LI 



Y OF CONGRESS, 



Chap. Copyright No. 

Shelf„3£„Sv5 



UNITED STATES OF AMERICA. 



I. 



; 



- 



THE SUN. 




SOLAR PROMINENCES. 



ELEMENTS 



OF 



Descriptive Astronomy, 



fr ftexUWook. 



BY 

HERBERT A. HOWE, A.M., Sc.D., 

PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF DENVER, AND 

DIRECTOR OF THE CHAMBERL1N OBSERVATORY ; 

AUTHOR OF "A STUDY OF THE SKY." 




4 



A 



SILVER, BURDETT AND COMPANY, 
New York , . . BOSTON . . . Chicago. 
i3 97 . 



M 



4? 



Copyright, 1897, 
By Silver, Burdett and Company. 



SEntbersitg $ress: 
John Wilson and Son, Cambridge, U.S.A. 



TO 

MR. HUMPHREY B. CHAMBERLIN, 

TO WHOSE MUNIFICENCE 
THE AUTHOR IS DEEPLY INDEBTED, 

E&ts Book 

IS GRATEFULLY DEDICATED. 



INTRODUCTION. 



THIS book is intended for the use of students who have a fair 
knowledge of elementary algebra and plane geometry, and is 
the outcome of several years of experience with classes of this sort. 
The author hopes that the volume will also be acceptable to more 
advanced scholars. 

The teacher may here be reminded of the fact that among the 
most urgent needs in the study of astronomy is the exercise of the 
geometric imagination ; that is, the faculty which forms mental 
pictures of the relative positions of planes and circles, and of the 
motions, real and apparent, of the celestial bodies. The initial 
step in astronomical instruction is to teach pupils the use of their 
eyes, — to insist that they observe the heavens and watch the celestial 
motions. Though they may be bewildered by such work at first, 
they will soon learn to delight in it, and will derive much profit from 
it. The earliest work in the line of observation is the study of the 
constellations. Acquaintance with the principal stars of the chief 
constellations visible in his latitude will prove a source of lifelong 
enjoyment to the pupil. Each student should keep a small blank 
book in w T hich to make sketches, and to record the results of obser- 
vations. 

The Star Maps, at the end of the volume, are on a generous scale, 
and include all stars not fainter than the fifth magnitude, from the 
north pole to 40 of south declination. By the use of these maps 
and star groups drawn on the blackboard, the teacher may greatly 
aid his pupils, who should copy the pictures and then find the corre- 
sponding objects in the sky. 

The use of a telescope adds much of interest to this study, espe- 
cially if the students are taught to manipulate it themselves, and to 
iv 



INTRODUCTION. V 

find by its aid the telescopic objects mentioned in Chapter XIV. 
Even a good opera-glass is very serviceable. The observation exer- 
cises given at times will be found very helpful, and will assist in the 
cultivation of the geometric imagination. A globe, with blackboard 
surface, may be useful in various ways, but as soon as the pupils 
have derived a geometric conception by its aid they should be led 
at once to transfer the mental picture to the heavens. 

It is also recommended that constant recourse be had to such 
periodicals as " Popular Astronomy," " Knowledge," etc., in order to 
follow the progress of astronomical research, month by month, and 
thus to supplement the text-book. The list of works, given in 
Appendix VII., is intended as a guide in the selection of an astro- 
nomical library. 

The optical principles of the telescope and spectroscope have 
been explained very simply, for students not familiar with descrip- 
tions of them in elementary text-books on physics. Especial 
attention has been paid to the Meridian Circle and to the Equatorial, 
because accurate knowledge of the positions and motions of the 
heavenly bodies depends chiefly on observations made with these 
instruments. 

The purely descriptive matter about the sun, moon, planets, etc., 
has been kept quite free from such statistics as the values of the 
masses of the planets, and the intensity of the pull of gravitation at 
the surface of each. The student should, however, learn the distance, 
diameter, time of revolution, and time of rotation of each planet. 
More extended data for purposes of reference are to be found in the 
Appendices. 

In this edition the results of the latest important investigations 
and discoveries have been stated. The work of the Lick Observa- 
tory, as set forth in the publications of the Astronomical Society of 
the Pacific, merits and has received much attention. The columns 
of astronomical periodicals have furnished a large amount of reliable 
information. 1 The author will welcome for a second edition any 
suggestions or corrections. 

The Exercises, which are a special feature of this book, and are 
placed at the end of each chapter, will be of great help to the pupils 

1 A good Star Atlas is a desideratum. Almost every school-book publishing house 
can furnish one. It will supplement the Star Maps at the end of this volume. 



VI INTRODUCTION. 

in reviewing the lessons, and also to the teacher in the work of the 
class-room. 

The Appendices contain, along with other useful material, questions 
for examination, topics for essays, and short reviews of a number of 
valuable works on Astronomy suitable for reference and general 
reading. 

A large number of the illustrations have never before appeared 
in any text-book. For many of the finer ones the author is in- 
debted to Prof. E. S. Holden, Prof. Wm. W. Payne, Mr. A. Cowper 
Ranyard, and Dr. E. E. Barnard. Prof. E. C. Pickering kindly 
furnished a fine set of lithographs, made from observations at Har- 
vard College Observatory, many of which have been reproduced. 
Prof. G. E. Hale has contributed photographs of the solar disturb- 
ance of July 15, 1892. Messrs. G. W. Saegmuller, of Washington, 
D. C, and J. A. Brashear, of Allegheny, Pa., supplied the pictures 
of some of the instruments. The author desires to state that he is 
specially indebted to his wife, Fannie Shattuck Howe, for her assist- 
ance in the preparation of the manuscript. 

University Park, Colorado, 
1896. 



CONTENTS. 



CHAPTER I. 

GENERAL SURVEY OF THE HEAVENS. 

Page 
Celestial Objects classified ; the Star Maps explained ; Names of the 
Constellations ; how to find the Constellations ; Hints on Constella- 
tion Study i 



CHAPTER II. 

APPARENT DAILY MOTION OF THE STARS. 

The Daily Motion ; the Celestial Sphere ; the Celestial Equator and 

Horizon ; Exercises 6 

CHAPTER III. 

THE TELESCOPE. 

Reflection and Refraction of Light; Lenses; Formation of an Image; 
Object-glass and Eyepieces ; Dispersion of Light ; Achromatism ; 
Refractors and Reflectors ; Equatorial Mountings ; Exercises .... 19 

CHAPTER IV. 

THE SUN. 

Its Distance and Diameter; how to Observe it with a Small Telescope; 
Photosphere; Faculae ; Spots; Solar Disturbances; Magnetic Storms; 
the Spectroscope ; Laws of Spectrum Analysis ; the Chromosphere : 
Prominences ; the Corona ; Light and Heat ; Constitution: Exercises . 36 

vii 



Vlll CONTENTS. 

CHAPTER V. 

THE EARTH. 

Page 
Dimensions; Latitude and Longitude; its Orbit; the Ecliptic; the Equi- 
nox and Solstices; the Zodiac; the Day; the Seasons; Precession 
and the Calendar; Aberration; Atmospheric Refraction; Twilight; 
Exercises 64 

CHAPTER VI. 

CELESTIAL MEASUREMENTS. 

Circles of Reference ; Parallax; Time; Solar and Sidereal Days ; Civil and 
Astronomical Days; Mean Solar and Sidereal Time; Standard Time, 
and its Determination by Means of a Meridian Circle and a Chrono- 
graph; Determination of Latitude and Longitude ; Exercises .... 87 

CHAPTER VII. 

THE MOON AND ECLIPSES. 

Distance; Diameter; Orbit; Period; Rotation; Librations ; Phases; Plains; 
Craters ; Mountains ; Water and Air ; Light and Heat ; Effect on the 
Weather; Eclipses, Solar and Lunar ; Exercises 108 

CHAPTER VIII. 

MOTIONS OF THE PLANETS. 

Their Orbits; Newton's and Kepler's Laws ; Aspects; Periods; Exercises. 133 

CHAPTER IX. 

MERCURY, VENUS, MARS, THE ASTEROIDS. 

Their Distance; Diameter; Revolution; Rotation; Phases; Satellites; 

Atmosphere ; Telescopic Appearance ; Physical Condition : Exercises. 143 

CHAPTER X. 

JUPITER, SATURN, URANUS, NEPTUNE. 

Their Distance ; Diameter; Revolution; Rotation; Discovery; Satellites; 

Atmosphere ; Telescopic Appearance ; Physical Condition ; Exercises. 164 



CONTENTS. IX 

CHAPTER XL 

COMETS AND METEORS. 

Page 
Comets, their Discovery ; Designation; Parts; Orbits; Appearances; Tails; 
Mass ; Light ; Spectra ; Fate. — Meteors, their Classes ; Paths ; Light 
and Heat; Constituents; Showers; Orbits, — Exercises 186 

CHAPTER XII. 

THE FIXED STARS. 

Number; Milky Way; Constellations; Names; Magnitudes; Dimen- 
sions; Distances; Clusters; Parallax; Spectra; Motions; Double and 
Multiple ; Variable ; Exercises 225 

CHAPTER XIII. 

THE NEBULAE. 

Various Forms; Spectra; Notable Ones; the Nebular Hypothesis; the 

Future of the Visible Universe 257 

CHAPTER XIV. 

THE CONSTELLATIONS IN DETAIL. 

The Greek Alphabet; Detailed Descriptions of the Constellations visible 
in the United States, with Tabular Lists of Prominent Double Stars, 
Clusters, Nebulae, Colored and Variable Stars 272 



APPENDICES. 

I. Names of Stars ! . 303 

IT. Astronomical Constants 304 

III. Planetary Data 305 

IV. Landmarks in the History of Astronomy . 307 

V. Topics for Essays 313 

VI. Queries for Use in Reviews and Examinations 315 

VII. List of Reference Books 320 



Index 327 

Star Maps . . 



LIST OF ILLUSTRATIONS. 



Page 

Frontispiece. The Solar Spectrum. 

Fig. i. Solar Prominences . . opposite i 

" 2. Revolution of the Sphere . . 7 

" 3. The Great Dipper and Polaris . 8 

Figs. 4-6. Diagrams illustrating Definitions 9 

" 7, 8. Apparent Daily Motion of the 

Stars 16 

Fig. 9. Apparent Daily Motion of the 

Stars 17 

" 10. Reflection by a Plane Mirror . . 19 

" 11. Reflection by a Concave Mirror . 20 

" 12. Refraction 20 

" 13. Refraction by Prisms .... 20 

" 14. Lenses 21 

" 15. Rays made Divergent . . . . 21 

" 16. Visual Angle 21 

" 17. Object and Image 22 

" 18. Lens as Eyepiece 23 

" 19. Object-Glass and Eyepiece . . 23 

' ; 20. Dispersion 25 

" 21. Dispersion Corrected .... 25 

" 22. Achromatic Object-Glass ... 26 

" 22. Portrait of Alvan Clark ... 26 

' ; 24. Eyepieces 27 

" 25. Path of Rays of Light in a Re- 
flector 27 

" 26. A Newtonian Reflector made by 

Brashear 2S 

' ; 27. Lord Rosse's Six-foot Reflector . 29 
" 2S. Scheme of an Equatorial Mount- 
ing 30 

" 29. A German Equatorial . . . . -1 

" 30. The Lick Telescope .... 32 

" 31. The Sun's Image on a Screen . 7,7 
" 32. Absorption of Light by the Sun's 

Atmosphere t>7 

" 23- Faculae observed Visually . . . ^ 

" 34. Sun Spot 4 o 

" 35- Lar £ e Su n Spot 4 t 

" 36. The Sun at the Time of a Spot 

Maximum ^2 

" 2>7- Photographs of the Disturbance 

of July 15, 1892 43 

X 



Fig. 3 S- 

" 39- 

" 40. 

" 4i- 

" 42. 

" 43- 

" 44- 

" 45- 

" 46. 

" 47- 

" 4 3. 

" 49- 



by 



Bra- 



53«- 
tigs. 53A 
Fig. 53,/. 

" 54- 

55- 
" ;6. 



Cyclonic Motion 

A Spectroscope made 
shear . . 

Plan of a Spectroscope 

Slit of a Spectroscope 

Production of Spectra 

Spectra 

Protrait of Kirch hoff 

A Geissler's Tube 

A Portion of the Solar Spectrum 

Correspondence of Bright and 
Dark Lines in Two Spectra 

Solar Prominences . . opposite 

A Spectroscope attached to a 
Telescope 

The Corona on July 29, 1878 . . 

The Corona, photographed on 
December 21, 1889 . . . . 

Illustrations of Schaeberle's The- 
ory of the Corona .... 
A Drawing of the Corona 

iy. Drawings of the Corona . 
A Photograph of the Inner Co- 
rona 

Electrical Appearances similar to 
the Corona 

Production of Heat by the Action 
of Gravity 

Direction of the Plumb-line . . 

How to find the Earth's Diame- 
ter . . . 

Latitude and Longitude 

The Astronomical Latitude 

An Ellipse 

Illustration of the Ecliptic . . 

The Obliquity of the Ecliptic 

The Ecliptic and the Equator 

The Sun's Daily Motion among 
the Stars „ . 

A Spinning Top 

Successive Positions of the Earth's 
Equator 

An Orange Half Submerged . . 



AGE 

44 

47 
47 
48 

49 
49 
5° 



5i 
53 J 



3° 

59 

61 

65 

66 

66 
67 
68 
69 
70 
70 

7i 
7i 

72 

72 



LIST OF ILLUSTRATIONS. 



XI 



Page 
Fig. 68. Locating the North Pole ... 74 

•' 69. The Midnight Sun 74 

" 70. Effect of the Slant of the Sun's 

Rays 75 

" 71. The Earth's Equatorial Ring . 75 

" 72. A Leaning Top 76 

" 73. Positions of the Axis of the Top 76 
" 74. The Precessional Motion of the 

Earth's Equator 77 

" 75. Path of North Celestial Pole 

among the Stars 78 

" 76. Different Kinds of Years ... 79 
" 77. Illustration of Aberration ... 81 

" 78. Refraction 82 

" 79. Gravity and the Earth's Rotation 82 
Figs. 80-83. Illumination of the Earth by 

the Sun 85 

Fig. 84. Circles of Reference .... 87 
41 85. Equator, Horizon, Meridian, 

etc 89 

" 86. Parallax 90 

" 87. Unequal Lengths of Apparent 

Solar Days ...... 91 

" 88. Variable Motion in Hour Angle 92 

" 89. A Standard Clock 95 

" 90. A Portable Meridian Circle . . 96 

" 91. A Reticle 97 

" 92. Motion of a Star's Image . . 97 

" 93. A Chronograph 99 

" 94. A Chronographic Record . . 100 
" 95. Determination of Latitude . . 100 
" 96. Latitude found by Observation 

of Polaris 10 1 

" 97. An Engineer's Transit . . . 102 

" 98. A Chronometer 103 

" 99. A Sextant ....... 104 

" 100. Orbits of the Earth and Moon . 10S 
" 101. Sidereal and Synodic Periods . 109 
" 102. Illustration of the Moon's Rotation 1 09 

Figs. 103, 104. Libration no 

Fig. 105. The Moon Illuminated . . . iit 
" T06. The Moon's Phases .... in 
" 107. The Moon : Photographed at 

Lick Observatory . . . . 112 
" 108. Skeleton Map of the Moon . . 114 
" 109. Conspicuous Craters of the 

Moon 115 

" no. The Crater Copernicus . . . 116 
'"' in. Portrait of Copernicus . . . 117 
" 112. The Terrestrial Crater Vesuvius 118 
" 113. The Lunar Apennines . . . 119 
" 114. The Crater Vendelinus : From 

Photograph 120 

" 115. Umbra and Penumbra of the 

Earth's Shadow 125 

" 116. Umbra and Penumbra of the 

Moon's Shadow 12; 



Fig. 117. 
« 11S. 


" 119. 


'' 120. 


" 121. 


" 122. 


" 123 


" 124. 


" I2 5 
" 126 


" 127 


" 128 


" 129 


" 130 


" 131 


" 132. 


" I 33- 


" 134 


" i35 
" 136 


" J 37 


" 13S 


" i39- 


" 140. 


" 141. 


" 142. 


" 143- 


" 144- 


" H5- 
" 146. 


" 147- 
" 148. 


" J 49- 



154. 



«6. 



59. 



160. 



161. 



Page 

Cross-section of a Shadow . . 125 
Beginning of a Total Lunar 

Eclipse 126 

Lunar Eclipse, Jan. 1888 opp. 126 
Path of Central Line of Eclipse, 

May 27, 1900 127 

Cause of Annular Eclipse . . 128 
Appearance of Sun during an 

Annular Eclipse .... 128 
Portrait of Sir Isaac Newton . 134 
Portrait of Kepler .... 135 
Equal Areas in Equal Times . 136 
Aspects of the Planets . . . 137 
Apparent M ovement of a Supe- 
rior Planet 139 

Relative Sizes of the Planets . 143 

A Transit of Venus .... 147 

The Black Drop 147 

Portrait of Galileo .... 148 

The Ring of Light .... 149 

Mars : Drawn by Barnard . . 151 

The Canals of Mars . . . . 155 

The Zone of Asteroids . . . 160 

Telescopic Experiment . . . 163 
Jupiter, as seen with the Lick 

Telescope 166 

Orbits of the Major Satellites . 168 

Phenomena of the Satellites . 169 
Markings seen with the Lick 

Telescope 169 

Jupiter and the Orbit of the 

Fifth Satellite 170 

Saturn, as seen with the Lick 

Telescope 172 

Different Positions of the Rings 174 

Old Drawing of Saturn . . . 175 

Portrait of Sir William Herschel 1 77 

Portrait of John Couch Adams 1S0 

Conic Sections 189 

Varieties of Orbits . . . . 1S9 
Orbits of some Comets of Jupi- 
ter's Family 191 

A Jet 192 

Companions of Brooks's Comet 193 

Development of a Tail . . . 194 

Comet's Tail. Type I. . . . 195 

Comet's Tail. Type II. . . 195 

Comet's Tail. Type III. . . 195 

Comet of 1528 JoS 

Comet of 186 1 20 ° 

The Great Comet of 1882 . . 203 
Nucleus of the Great Comet of 

1SS2 204 

Swift's Comet : Photographed 

by Barnard 205 

Brooks's Comet : Photographed 

by Barnard 207 






Xll 



LIST OF ILLUSTRATIONS. 



F 


ig. 


162. 




a 

a 

u 

a 
it 


163. 
164. 

165. 

166. 
167. 
16S. 
169. 
170. 

171. 
172. 



« 175. 

" 176. 

' : I 77- 

' ; 17S. 



Page 
Meteor seen at Bassein, Bur- 

mah 210 

A Meteorite 212 

The Canyon Diablo Meteorite . 213 
Relative Frequency of Meteors 

in the Morning and Evening . 215 

The Radiant 216 

The Orbit of the August Shower 218 

Capture of the Leonids . . . 220 

Stars Visible to the Naked Eye 224 
The Verkes Telescope at the 

World's Fair, 1893 . ■ ■ ■ 226 

A Portion of the Milky Way . 227 

Plant-like Structure .... 228 

The Great Cluster in Hercules 232 
The Cluster Omega Centauri : 

Photographed by Dr. Gill at 

the Cape of Good Hope . . 233 

Stellar Parallax 234 

Method of observing Stellar 

Parallax 234 

Relation of Parallax to Distance 235 

Proper Motions of the Pleiades 240 



Fig. 179. 

•' 180. 

" 1S1. 

il 1S2. 

" 183. 

» 184. 

" 185. 

" 1S6. 

» 1S7. 

' ; 1S8. 

" 1S9. 

i; 190. 

i( 191. 

" 192. 



193. 

194. 
195- 



Page 

Double Stars 242 

A Spectroscopic Binary . . . 244 

Multiple Stars 245 

Portrait of Tycho Brahe . . 247 

Tycho's Star in Cassiopeia . 248 

How to find Algol .... 249 

Y Cygni 250 

Real Velocity of a Star . . . 254 
The Pleiades : Photographed 

by Roberts 258 

The Trifid Nebula .... 260 
The Nebula in Andromeda: 

Photographed by Roberts . 26 1 
The Nebula in Orion : Drawn 

by Bond 262 

The Ring Nebula in Lyra : 

Drawn by Bond 264 

The Spiral Nebula in Canes 

Venatici : Photographed by 

Roberts 265 

Portrait of La Place . . t . 266 

The Star Finder 295 

The Declination Circle . . . 295 




ELEMENTS 

OF 

DESCRIPTIVE ASTRONOMY, 



CHAPTER I. 

GENERAL SURVEY OF THE HEAVENS. 

" The sky 
Spreads like an ocean hung on high, 
Bespangled with those isles of light 
So wildly, spiritually bright. 
Who ever gazed upon them shining, 
And turned to earth without repining, 
Nor wished for wings to flee away, 
And mix with their eternal ray ? " 

Byron. 

1. The Fixed Stars. — The fixed stars are points of light, of vari- 
ous degrees of brightness, which bestrew the sky. They are called 
fixed, because, as seen with the naked eye, they do not change their 
relative positions from year to year. 

In the earliest ages men divided them into various groups, which 
we call constellations ; the appearance of each of these constella- 
tions is almost the same to-day as when it was named by the 
ancients. 

A star just visible to an average eye is said to be of the sixth 
magnitude ; one a little brighter is said to be of the fifth magnitude, 
and so on, a few of the brightest being called first magnitude stars. 
All fixed stars are at inconceivably great distances from us. 

2. The Planets. — The word planet is derived from a Greek word 
meaning "a wanderer." The designation is applied to certain star- 
like objects which appear to move among the fixed stars. The 

i 



2 DESCRIPTIVE ASTRONOMY. 

brightest ones have received the names of ancient divinities, as 
Jupiter, Saturn, and Venus. They revolve about the sun in paths 
nearly circular ; the earth is considered one of them. 

3. The Moon. — This familiar object revolves about the earth in 
27J days. It is the nearest of the celestial bodies, being a little 
less than a quarter of a million of miles away. It belongs to the 
class of objects known as satellites, which revolve about the planets, 
held fast by their attractive force. 

4. The Sun. — The sun, like the moon, is so familiar that no 
particular description of it is needed here. Though its distance 
from the earth is nearly 93,000000 miles, it is very much nearer to 
us than any one of the fixed stars. To its abounding light and 
heat we owe the preservation of our lives, the maintenance of our 
vigor, the physical comforts which we enjoy, and the marvellous 
beauties of nature. 

5. Comets. — The word "comet" is derived from a Greek word 
meaning " long-haired." Comets are usually invisible to the naked 
eye, but sometimes attain great splendor and beauty. In ancient 
and mediaeval times their appearance was usually regarded as a dire 
omen. Even in 1861 it was rumored in Italy that the great comet 
of that year presaged the death of Pope Pius IX. 

6. Meteors. — These are evanescent objects which flash across 
the sky, and usually fade from sight in a few seconds. Occasionally 
they are so brilliant as to be seen in broad daylight, and are 
accompanied by terrific detonations. 

7. Nebulae. — Nebulae, as their name implies, are cloudlike in 
appearance. A few of them are conspicuous enough to be faintly 
seen without telescopic aid, and appear as feeble patches of light 
on the dark background of the sky. They are large and diffuse 
masses of matter, at vast distances from us. 

8. The Star Maps. — In the maps at the end of this book, the 
magnitudes are indicated very simply ; especial care has been taken 
in drawing the dotted lines connecting the stars in the constellations, 
so that figures easily remembered may be obtained. Careful direc- 
tions are given for learning the constellations, together with lists of 
telescopic objects, most of which are within the power of a three- 
inch telescope. Stars of the first magnitude are indicated by heavy 
black dots. Those of the fifth magnitude are represented by small 



GENERAL SURVEY OF THE HEAVENS. 3 

dots. A star of the second magnitude has two short arms project- 
ing from a central dot. Stars of the third and fourth magnitude 
have, respectively, three and four projecting arms. The figures at 
the top and bottom of each map, except the first, denote the right 
ascensions of the stars ; those at the sides indicate the declinations. 
On Map I. right ascensions are given around the circumference, 
declinations along a diameter. These terms are explained in § 122. 
They are hot needed in learning the constellations. Each con- 
stellation is bounded by a heavy dotted line, and its name is printed 
in large letters. The proper names of the brightest stars, such as 
Sirius, Vega, etc., are given. Most of the stars are marked by 
letters or numbers. The name of such a star is formed by adding 
to its letter the Latin genitive of the name of the constellation in 
which it lies. Thus the star m in Orion is called m Orionis. 

9. Names of the Constellations. — The names of the constellations 
shown on the Star Maps are given in the table below. The Greek 
alphabet is found in § 405. 



Latin (Nominative) 

An-drom'-e-da 

A-qua'-ri-us 

Aquila (Ak'-wi-la) 

Ar-go Na-vis 

Aries (A'-ri-ez) 

Au-ri'-ga 

Bootes (Bo-d'-tez) 

Cam-el-o-par'-dus 

Can-cer 

Canes Venatici (Ka'-nez Ve-nat'-i-si) 

Ca-nis Ma-jor 

Ca-nis MT-nor 

Cap-ri-cor'-nus 

Cassiopeia (Kas-si-o-pe'-ya) 

Centaurus (Sen-taw'-rus) 

Ce'-phe-us 

Ce-tus 

Co-lum'ba 

Coma Berenices 

Co-ro'-na Bo-re-a'-lis 

Corvus 

Cra-ter 

Cyg'-nus 

Del-phi'-nus 

Dra-co 



Latin (Genitive) 

Andromedae 

Aquarii 

Aquilae 

Argus 

Arietis 

Aurigae 

Bootis 

Camelopardi 

Cancri 

Can. Ven. 

Canis Majo'ris 

Canis Mino'ris 

Capricorni 

Cassiope'iae 

Centau/ri 

Cephei 

Ceti 

Columbae 

Comae Berenices 

Coronas Borea/lis 

Corvi 

Cra'teris 

Cygni 

Delphi ni 

Draco'nis 



English 
Andromeda 
The Water Carrier 
The Eagle 
The Ship 
The Ram 
The Charioteer 
The Bear Keeper 
The Camelopard 
The Crab 
The Hunting Dogs 
The Great Dog 
The Little Dog 
The Goat 

The Lady in the Chair 
The Centaur 
Cepheus 
The Whale 
The Dove 

The Hair of Berenice 
The Northern Crown 
The Crow 
The Cup 
The Swan 
The Dolphin 
The Dragon 



DESCRIPTIVE ASTRONOMY. 



Latin (Nominative) 


Latin (Genitive) 


English 


Equuleus (E-kwu'-le-us) 


Equu'lei 


The Little Horse 


E-rid'-a-nus 


Eridani 


The River 


Gem'-i-ni 


Gemino'rum 


The Twins 


Her'-cu-les 


Herculis 


Hercules 


Hy'-dra 


Hydrae 


The Snake 


La-cer'-ta 


Lacertae 


The Lizard 


Le-o 


Leo'nis 


The Lion 


Le-o Mi-nor 


Leo'nis Mino'ris 


The Little Lion 


Le-pus 


Lep'-o-ris 


The Hare 


Li-bra 


Librae 


The Scales 


Lu-pus 


Lupi 


The Wolf 


Lynx 


Lyncis 


The Lynx 


Ly'-ra 


Lyrae 


The Harp 


Monoceros (Mo-nos'-e-ros) 


Monocero'tis 


The Unicorn 


Oph-i-u'-chus 


Ophiuchi 


The Serpent Bearer 


O-ri'-on 


Orionis 


Orion 


Peg'-a-sus 


Pegasi 


The Winged Horse 


Per'-se-us 


Persei 


Perseus 


Pisces (Pis'-sez) 


Piscium 


The Fishes 


Pis'-cis Aus-tra'-lis 


Piscis Austr. 


The Southern Fish 


Sagitta (Sa-jit'-ta) 


Sagittas 


The Arrow 


Sagittarius (Saj-i-ta'-ri-us) 


Sagittarii 


The Archer 


Scor'-pi-6 


Scorpii 


The Scorpion 


Sculp-tor 


Sculpto'ris 


The Sculptor 


Scu-tum 


Scuti 


The Shield 


Serpens (Ser'-penz) 


Serpentis 


The Serpent 


Sextans (Seks'-tanz) 


Sextantis 


The Sextant 


Taurus (Taw'-rus) 


Tauri 


The Bull 


Triangulum (Tri-ang'-gu-lum) 


Trianguli 


The Triangle 


Ur'-sa Ma-jor 


Ursas Majo'ris 


The Great Bear 


Ur'-sa Mi-nor 


Ursas Mino'ris 


The Little Bear 


Vir'-go 


Vir'-gTnis 


The Virgin 


Vul-pec'-u-la 


Vulpeculae 


The Fox 



10. How to Find the Northern Constellations. — The constellations 
visible in the northern sky are to be found on Map I. The appear- 
ance of these constellations at 8 o'clock on any evening may be 
found by using the dates around the circumference of the map. If 
the aspect of the northern sky on April 1st, for instance, is desired, 
hold the map so that the date April 1st shall be uppermost. Face 
the north and hold the map up toward the sky. 

The Great Dipper, which is a portion of the constellation of the 
Great Bear (Ursa Major), can then be found readily. 

After the Great Dipper has been fixed in mind, the Pole Star 
can be located by the help of Fig. 3. Cassiopeia is on the opposite 



GENERAL SURVEY OF THE HEAVENS. 5 

side of the Pole Star from the Great Dipper, and at about the same 
distance from it. The five brightest stars in this constellation form 
a straggling W and are quickly discovered. 

Beginning at the Pole Star, one can then trace out the Little Bear 
(Ursa Minor) with the assistance of the map. 

When the outlines of these constellations have been learned thor- 
oughly, there will be very little difficulty in becoming acquainted 
with the adjacent ones. 

11. How to Find the Constellations in the South. — These constella- 
tions are pictured on Maps II. -V. 

The stars underneath any particular date at the top of the map 
are to be seen in the south at 8 P. M. on that date. For example, 
Map II. shows that on February 16th Orion is in the south at 8 P. M. 
In winter, Orion is the best southern constellation to learn first, 
because of its conspicuousness. 

In spring, Leo (see Map III.) is recommended as a starting 
point; in summer, Scorpio (see Map IV.) will answer the same 
purpose ; it will be seen low down in the south. For autumn, 
Pegasus (see Map V.) is available. Four of the principal stars of 
this constellation form a large square. 

12. Hints on Constellation Study. — When trying to learn any 
particular constellation, the student should find from the maps about 
half a dozen or a dozen of its principal stars. The configuration of 
these should be impressed upon the mind so thoroughly that a 
drawing showing their relative positions can be made without 
looking at the maps. With this picture well in mind, the student 
may confront the sky. It is well to note the brightness and color 
of any first magnitude star in the constellation, and to learn its 
name. In many cases one can trace in a constellation a resemblance 
to the object after which it is named. Orion, for instance, resembles 
the figure of a man. The best way to secure a thorough acquaint- 
ance with the constellations which have been learned is to draw 
them from memory frequently, and to look at the heavens whenever 
occasion offers. 

The descriptive matter in Chapter XIV. will be found useful in 
this work. 



DESCRIPTIVE ASTRONOMY. 



CHAPTER II. 

APPARENT DAILY MOTION OF THE STARS. 

" The sad and solemn night 
Hath yet her multitude of cheerful fires ; 
The glorious host of light 
Walk the dark hemisphere till she retires ; 
All through her silent watches, gliding slow, 
Her constellations come, and climb the heavens, and go." 

Bryant. 

13. The Daily Motion. — The most casual observer cannot fail to 
notice that the majority of the visible stars daily rise, travel across 
the sky, and set. 

The Greek philosopher, Pythagoras, is said to have taught that 
the thousands of fixed stars which stud the sky were set in a crystal 
sphere, which, by its daily revolution, carried them around the earth 
as a centre. 

By this theory the apparent daily motion of the stars was ex- 
plained very simply and accurately, so far as the unassisted eye 
could judge. 

14. Cause of the Motion. — When a passenger train, which has 
been standing near a motionless freight train, starts without per- 
ceptible jar, a passenger looking at the freight train has the impres- 
sion at first that the latter train is moving, and his own standing 
still. A person on the deck of a steamer, which is slowly turning 
about in a harbor, sees the objects on shore apparently revolving 
about him. If a visitor to an astronomical observatory looks upward 
at the dome while it is being turned, the floor on which he stands 
seems to be revolving. Similarly, the apparent daily revolution of 
the heavens is an illusion. 

The earth turns upon its axis once a day, but makes the rotation 
without noise or jar, so that the observer is unconscious of it, and is 
led to think that the earth is at rest, and that the sky moves. 

15. The Celestial Sphere. — As one looks by night at the heavenly 
bodies, they all seem to lie on the surface of an immense dome. 



APPARENT DAILY MOTION OF THE STARS. 7 

Were the earth to vanish suddenly, the observer would seem to be 
in the centre of a hollow star-spangled sphere. 

The distance from his eye to any celestial object he could not 
tell. But his reason would quickly declare that the stars might 
really be at widely different distances from him, while they appeared 
to be upon a spherical surface which lay beyond them all. Each 
star would seem to be situated where a straight line drawn from his 
eye through the star, met the spherical surface. This imaginary 
spherical surface is called the celestial sphere. 

16. Radius of the Celestial Sphere. — Astronomers find it conven- 
ient to assume that the radius of the celestial sphere is infinite, that 
is, too great for human comprehension. Were it possible for a man 
to take his stand upon the celestial sphere and look back at the vast 
assemblage of worlds which we call the physical universe, their 
combined mass would appear to him to be a mere point of light, 
which would be situated at the centre of the sphere. Hence it is 
evident that the observer's eye, or any point on the earth's surface, 
or the earth's centre, or the sun's centre, may be considered without 
palpable error as the centre of the celestial sphere. 

17. Revolution of the Sphere. The Poles. — Imagine that the 
earth's axis is prolonged until it strikes the celestial sphere at two 
opposite points, called the north and south celestial poles, respect- 
ively. The heavens appear to revolve about an axis drawn from 
one celestial pole to the other. 




Fig. 2. 



In Fig. 2, P is the north pole of the earth and P' the south pole ; 
O is the position of an observer upon its surface ; AB is drawn 
through O parallel to PP r . Now there is no fixed star which is 



8 DESCRIPTIVE ASTRONOMY. 

known to be nearer to us than 20,000000,000000 miles. To give 
greater definiteness to our ideas, conceive the radius of the celestial 
sphere to be 20,000000,000000 miles, and its centre to be at the 
earth's centre. Prolong AB and PP', until they strike the surface 
of the sphere : suppose the places where they strike to be marked by 
brilliant points of light. So enormous is the radius of the sphere 
when compared with the distance between AB and PP", that to an 
observer on the earth an extremity of AB would seem to coincide 
with the corresponding extremity of PP', even if the observer's eye 
were assisted by the most powerful telescope of modern times. 

We may therefore consider the sky as revolving on AB as an 
axis, and may state the following principle. 

The celestial sphere appears to revolve on an axis drawn from the 
eye of the observer to either pole of the celestial sphere. 

18. Location of the North Celestial Pole. — Nearly every one is 
familiar with the configuration of seven bright stars which is called 



h±_ + 



7 






Fig. 3. — The Great Dipper and Polaris. 



"The Great Dipper." It is represented in Fig. 3. The two stars 
in the bowl of the dipper which point nearly to the Pole Star 
(Polaris) are called " The Pointers." The distance from Polaris to 
the nearer one of the Pointers is about five times the distance be- 
tween the latter. The position of the pole is shown in the figure ; 



APPARENT DAILY MOTION OF THE STARS. 



9 



it lies very near a line drawn from Polaris to Mizar; its distance 
from Polaris is one fourth of the distance between the Pointers. 

19. Definitions. — If a straight line does not lie in a plane, but 
meets it at some point, the point is called the foot of the line. A 
straight line is perpendicular to a plane when it is perpendicular to 
every straight line that can be drawn in the plane through its foot. 

The corner of a room, where two walls meet, is a line perpendic- 
ular to the plane of the ceiling, or of the floor. 



F r 


K- ' 


E L 



Fig. 4. 



Fig. 5. 



A straight line is parallel to a plane when they cannot meet, 
however far they may be extended. 

When two planes meet, their line of intersection is called the 
edge of the angle which they make with each other. The planes 
AC and CF meet in the edge BC. At any point, H, in the edge, 
two perpendiculars to the edge are drawn, one lying in each plane. 
The angle GHK made by these perpendiculars measures the angle 
between the planes. 

To find the angle which a line, prolonged if necessary, makes 
with a plane which it meets, a per- 
pendicular to the plane is dropped 
from some point A in the line : the 
foot of the perpendicular is then 
joined with the foot of the original 
line. The angle which the last line 
drawn makes with the original line is 
the angle sought. In the figure, 
AB is the original line, AC the per- 
pendicular, BC the joining line, and ABC the angle. 

20. The Celestial Equator. — The equator of the earth is an imagi- 
nary line encircling it, midway between the poles. The plane of the 




Fig. 6. 



IO DESCRIPTIVE ASTRONOMY. 

equator extended indefinitely cuts the celestial sphere in a circle, 
which is called the celestial equator. Since the plane of the ter- 
restrial equator is perpendicular to the earth's axis, the plane of 
the celestial equator is perpendicular to the axis of the celestial 
sphere. 

21. The Horizon. — The word ''horizon" is commonly used to 
designate the line where the earth and sky appear to meet. But 
astronomers use the word in a different sense. If the observer holds 
a plumb-line so that it hangs vertically, any plane surface, like a 
book-cover, when held so that it is perpendicular to the plumb-line, 
will represent a portion of the plane of the horizon of the place of 
observation. If the flat surface of the book-cover be extended hori- 
zontally in all directions until it reaches the sky, the plane thus 
formed is called the plane of the horizon. This plane cuts the 
celestial sphere in a circle called the horizon. 

22. The Zenith, for any place on the earth, is the point where 
a plumb-line prolonged upward strikes the celestial sphere. 

23. The Nadir is the point where the plumb-line prolonged 
downward strikes the celestial sphere. 

The zenith and nadir are those points on the celestial sphere 
which are most remote from the horizon. 

In the language of geometry, they are called the poles of the 
horizon. 

EXERCISES. 

24. I. Narrate some personal experience of illusory motion 
similar to those mentioned in § 14. 

2. The diameter of the earth is about eight thousand miles. 
What is the greatest possible distance between AB and PP' in 
Fig. 2? 

3. The observer is on the terrestrial equator. 

(a) What is the distance in miles between AB and PP' in Fig. 2 ? 

(b) If he walked toward either pole, and the earth were a 
perfect sphere, would this distance increase or decrease? 

4. If another line were drawn through O in Fig. 2, making an 
angle of io° (one ninth of a right angle) with AB, would AB and 
the new line, when prolonged, meet the celestial sphere at the same 
points apparently? 



APPARENT DAILY MOTION OF THE STARS. I I 

[If there be any doubt in the student's mind concerning this, he 
should take two straight sticks, and put an end of one in contact 
with an end of the other, so that the sticks make an angle of io° 
with each other. Then, placing his eye near the vertex of the 
angle, he can look along each stick at the sky.] 

5. If the nearest fixed star were 20,000000,000000 miles from 
us, how many years would it take its light, travelling 186,330 miles 
per second, to reach us? Ans. 3.4+ years. 

6. Look at the Great Dipper and Polaris in the sky, and draw a 
map of them ; first make a dot for Polaris and draw a line below it 
to represent the horizon. Draw another line through Polaris per- 
pendicular to the horizon line. Draw the Dipper, showing its 
position with reference to these lines, and state the hour at which 
your observation was made. 

7. The observer, facing north, notices that the Dipper is below 
the pole. 

(a) Will the Dipper appear, on account of the earth's rotation, 
to move towards his right hand, or towards his left? 

(b) What would be the direction of motion of the Dipper rf it 
were above the pole, near the zenith? 

(c) What direction (up or down), if at the right of the pole? 

(d) What direction, if at the left? 

8. If a line be drawn from your eye to each of the two 
" Pointers" in the Dipper, the two lines make an angle of about 5 . 
Astronomers commonly say that the distance between the Point- 
ers is 5 . Estimate the distance from the Pole Star to the Pointer 
nearest to it. 

9. Suppose that the earth's centre is fixed in the centre of the 
celestial sphere, and that the earth rotates on its axis. Imagine 
all the stars to be fixed on the surface of the celestial sphere. 
If the earth's axis were then tipped a few degrees, would the north 
celestial pole remain at the same point among the stars as before? 
Would the position of the celestial equator be changed? 

10. If, instead of tipping the axis of the earth, as in the pre- 
ceding exercise, the earth were moved toward some point on the 
celestial equator and were placed 1,000000 miles away from its 
former position, the new direction of its axis being parallel to its 
former direction, would the new celestial poles appear coincident 



12 DESCRIPTIVE ASTRONOMY. 

with the former celestial poles? Would the old and new celestial 
equators coincide? 

11. Conceive that the earth, in exercise 9, is rotating about a 
straight wire stretched from the north celestial pole to the south 
celestial pole. If it were slid along this wire to a place 10,000000 
miles from its first position, would the new celestial equator appear 
to coincide with the old? 

12. The earth makes its annual journey around the sun in a 
path nearly circular. 

(a) If the earth's axis at every instant during the year were 
parallel to its position at every other instant, would the celestial 
poles change their position during the year? 

(b) Would the celestial equator change its position? 

[The earth's axis remains almost parallel to itself during a year. 
Its deviations will be explained later. So far as naked-eye obser- 
vations are concerned, it may be considered as remaining parallel 
to itself.] 

13. Take a ball or an orange to represent the earth. Mark on 
it the north and south poles and the equator. Find from a 
geography or other source the latitude of your place of observation 
to the nearest degree, and locate the place on the ball. (If the 
latitude were 45 , the place would be halfway between the pole and 
the equator.) Take a flat stiff card, and lay one of its surfaces 
against the ball, at the point representing the place of observation. 
Fasten the card by a pin thrust through it into the ball at the point 
of contact. The pin should point toward the centre of the ball. 
The surface of the card will then represent the plane of the horizon 
of the place, and will be tangent to the spherical surface of the ball. 

(a) Is your horizon parallel or perpendicular to the earth's 
axis? 

(b) Is it parallel to the earth's equator? 

(c) If you were at some point on the earth's equator, would 
your horizon plane be perpendicular to the earth's axis, or parallel 
to it? 

(d) If you were at the north pole, would your horizon plane be 
parallel to the earth's axis, or perpendicular to it? 

(e) As the earth turns on its axis, does the inclination of the 
axis to the plane of your horizon change? 



APPARENT DAILY MOTION OF THE STARS. 1 3 

14. If the polar axis of the ball in the last exercise be prolonged 
both ways, which prolongation (north or south) would pierce the 
plane of the card representing your horizon plane? If the ball be 
held so that the card is horizontal, what points on the celestial 
sphere would the pin strike if prolonged indefinitely each way? 

Remark. — In obtaining the answers to the following exercises, 
the apparatus described in exercise 13 can be used; but it would be 
much better to imagine the earth itself, with its poles and equator, 
and with horizon planes touching it at different points. The student 
should make every endeavor to picture to himself the realities of 
nature, ratJier than the apparatus or the geometrical diagrams used 
to explain principles. Wherever possible, he should observe the celes- 
tial motions about which he studies. 

15. If you were at the north pole, would your horizon plane be 
parallel to the plane of the earth's equator, or perpendicular to it? 

If you lived at the equator, would your horizon plane be parallel 
to the earth's equator, or perpendicular to it? 

16. (a) If one man were at the north pole and another at the 
south, would their horizon planes be parallel? 

(b) If one man were at the north pole and another at any point 
of the equator, would their horizon planes be perpendicular to each 
other? 

(c) Could the horizon planes at two points on the equator be 
parallel? 

(d) Could they be perpendicular to each other? 

Remark. — In answering questions about the rising and setting 
of the stars, the student should remember that the horizon of the 
place of observation seems motionless, while the heavens appear 
to revolve about a line drawn from the observer's eye to either 
celestial pole. 

17. The radius of the celestial sphere is considered infinite. 
The observer is at the north pole. 

(a) Does his horizon coincide with the celestial equator? 

(b) Is the north celestial pole at his zenith? 

(c) Could he see a star which lay between the south celestial 
pole and the equator? 

(d) Would any star visible to him set within 24 hours? 

(e) Every star in the sky would appear to describe a circle in 



14 DESCRIPTIVE ASTRONOMY. 

24 hours. Would the planes of these circles be parallel to his 
horizon ? 

(/) If the sun were always north of the celestial equator, would 
night ever come for him? 

18. A man lives at some point on the equator. 

(a) Does his horizon coincide with the celestial equator? 

(b) Do the celestial poles lie on his horizon? 

(c) Is the plane of the celestial equator perpendicular to the 
plane of his horizon? 

(d) If the celestial equator were drawn as a line of light on the 
celestial sphere, would it pass through his zenith? 

(e) Would the celestial equator cut his horizon? 

(_/") If so, at what points (north, south, east, or west)? 

(g) How great a portion of the celestial equator would be 
visible at any instant? 

(/i) If a star rose at the east point of the horizon, at what point 
would it set? 

(i) If a star rose a little north of the east point of the horizon, 
would it set a little north of the west point, or a little south of that 
point? 

(/) If a star rose at a point half way between the south and 
east points of the horizon, where would it set? 

{k) Where would Polaris rise and set? 

(/) For how many hours would a star be above the horizon, 
and for how many below? 

Remark. — To assist in forming clear ideas about the answers 
to the questions in exercise 19, the scholar may take an orange, 
through which a knitting-needle has been thrust, to represent the 
celestial sphere ; on it a circle may be drawn to represent the celes- 
tial equator ; other circles may be drawn parallel to this one. The 
orange may then be half submerged in water, as shown in Fig. 6y. 
The surface of the water will represent the plane of the observer's 
horizon, and the upper half of the orange the visible heavens, the 
observer being supposed to be at the centre of the orange. 

19. The observer is located somewhere between the north pole 
and the equator, say at 40° north latitude. 

(a) Is his horizon plane parallel or perpendicular to the celestial 
equator? 



APPARENT DAILY MOTION OF THE STARS. 1 5 

(b) Does the axis of the celestial sphere make an oblique angle 
with his horizon plane? 

(c) Does a line from the north celestial pole to the observer's 
eye make a right angle with his horizon plane, or an acute angle? 

(d) If a plane be passed through the observer's eye, perpen- 
dicular to the line last mentioned, will it be parallel to the plane of 
the earth's equator? 

(e) The plane just passed through the observer's eye intersects 
the horizon in a line : what direction (north and south, or east and 
west) does that line have? 

(/") The plane mentioned, when extended in all directions to 
the celestial sphere, will cut a circle on it, half of which is above 
the horizon ; if this semicircle could be seen as a line of light 
on the celestial sphere, would it lie north of the zenith, or south 
of it? 

{g) Would this semicircle coincide with half of the celestial 
equator? 

(k) With extended arm and forefinger point to the east point of 
the horizon ; swing your arm in such a way that your forefinger will 
point to the celestial equator, as it runs from the east point of the 
horizon to the west point. 

(z) Similarly trace the circle which Polaris describes in a day. 

(7) Similarly trace the circle which a star io° from the north 
celestial pole would describe in a day. 

(k) As seen from your home, does the bowl of the Great Dipper 
ever set? 

(/) If a star rises half way between the north and east points 
of your horizon, at what point of the horizon will it set? 

(m) If it could be watched for 24 hours, would it be above the 
horizon just 12 hours, or more, or less? 

(») If a star rose at the east point of the horizon, would it be 
above the horizon just 12 hours? 

(0) If a star rose half way between the east and south points of 
the horizon, would it be above the horizon more or less than 12 
hours? 

(/) If a star is between the north celestial pole and the celestial 
equator, will it be above your horizon more or less than half a day 
at a time? 






i6 



DESCRIPTIVE ASTRONOMY. 



(g) If a star is between the south celestial pole and the celestial 
equator, will it be below your horizon more or less than half a day 
at a time? 

(V) Point your finger at the south celestial pole. 

(s) Could a star be so near the south celestial pole that it could 
not be seen from your home? 

20. State which one of the following diagrams represents the 
apparent daily motion of the stars as seen from the north pole. 
Which, as seen from the equator. Which, as seen from a place in 
latitude 40 north. 




— ^.v i. I ... — - 



Fig. 7. 




Fig. 8. 



APPARENT DAILY MOTION OF THE STARS. 



•7 




Fig. 9. 



21. Find, with the teacher's aid if necessary, some bright planet, 
which will be visible during the time you expect to devote to the 
study of this book : on a moonless night, make a map showing its 
position with reference to the neighboring bright stars. Preserve 
the map, and note on it the planet's position among the stars from 
week to week. 

22. Take your seat in a dark room, before a south window. 
Adjust your head so that by looking with one eye just past the 
western sash of the window you will see a star. Hold your head 
steady until you see the star disappear behind the sash. The farther 
you are from the window, the easier the observation will be. If you 
have a good opera-glass or spy-glass, by fastening it so that it will 
point to the southern portion of the sky, you can observe the motion 
of the stars more easily. Near the north celestial pole the apparent 
motions of the stars are too slow to be observed satisfactorily in 
this way. 

23. At your first opportunity, early in the evening, draw a map 
similar to that required in exercise 6. Before retiring for the night, 
look again, and draw another map of the Dipper. 

(d) Does a comparison of these maps show a movement of this 
group ? 

(b) If you look at the face of a watch held between your eye 
and the north celestial pole, will its minute hand move around in the 
same direction as the Dipper? 



I 8 DESCRIPTIVE ASTRONOMY. 

24. Some evening, notice the hour and minute when some star, 
easily recognized again, is near the eastern horizon. After about two 
weeks, at the same hour and minute, look for the star. Is it nearer 
the horizon than before? Try the same experiment, at the same 
time, with a star near the western horizon. From these observations 
determine whether a star will rise and set earlier than at present, a 
month from now, or later. 

25. Find from an almanac or diary the date of the next new 
moon. An evening or two thereafter, look in the west for the moon, 
in the evening twilight. When first seen, draw a sketch of it and 
date the sketch. Sketch its form every clear evening thereafter until 
it does not rise before your bedtime : then look for it in the morning, 
and continue sketching it, if possible, until the next new moon. 

(a) While making these sketches, did you notice that the moon 
moved westward among the stars ? 

(b) Did you see the dark part of the moon? 

(c) When the moon was a slender crescent did the cusps or 
" horns" of the crescent point toward the sun? 

(d) When the moon was full (a complete circle of light) did it 
rise at about the time of sunset? 

(e) Did you ever see a star between the cusps of the moon? 

(f) Did you ever see the moon occult a star, that is, hide it from 
view? 

26. On some moonless night, find the Milky Way, which is a 
broad band of hazy light. 

(a) Are there any dark places in it? 

{b) Are there any brilliant spots in it? 

(c) Does it run through the Great Dipper? 



THE TELESCOPE. 19 



CHAPTER III. 

THE TELESCOPE. 1 

" Through thee will Holy Science, putting off 
Earth's dusty sandals from her radiant feet, 
Survey God's beauteous firmament unrolled 
Like to a book new-writ in golden words, 
And turn the azure scroll with reverent hand, 
And read to men the wonders God hath wrought." 

Anon. 

25. Refractors and Reflectors. — There are two kinds of telescopes, 
called respectively refractors and reflectors. Opera-glasses and spy- 
glasses belong to the former class. 

Reflectors are rarely seen, except in connection with astronom- 
ical observatories. The principal portion of one of these is a large 
curved mirror, which reflects the rays of light coming from the 
object viewed, in a manner to be explained hereafter. 

In order to understand the action of a telescope, one must be 
acquainted with a few elementary principles 
of optics, which we proceed to unfold. 

26. Reflection by a Plane Mirror. — In 
the figure, DC is a ray of light striking the 
plane mirror AB at the point C ; it is re- 
flected along the line CF. EC is perpen- 
dicular to AB. The angle DCE, which the 
incident ray makes with the perpendicular C 

to the mirror at C, the point of incidence, Fi §- 10. — Reflection by a 

,j ix- -j c -i 1 -r-r-T- Plane Mirror. 

is t/ie angle of incidence. Similarly ECr 

is the angle of reflection. The angle of incidence is equal to the angle 

of reflection. 

1 It is well to illustrate this chapter as thoroughly as possible by experiments with 
lenses and mirrors. When a class is pressed for time, the more mathematical portions 
of the chapter may be omitted, or simply explained by the teacher. If an equatoriallv 
mounted telescope is not available, a rude wooden model of it may easily lie made, and 
will be of service. 




20 



DESCRIPTIVE ASTRONOMY. 



27. Reflection by a Concave Mirror. — A concave mirror may be 
considered as made up of a very large number of minute plane 
mirrors. If a system of parallel rays strikes the surface of a con- 
cave spherical mirror, each ray will be reflected at its point of 
incidence, in accordance with the principle of § 26. The reflected 
rays will converge and will meet (almost exactly) at a point called 
the focus (Fig. 11). Opticians make their mirrors deviate slightly 
from a true spherical form, in such a way that all rays are brought 
accurately to a focus. 

Note. — In §§ 28-34 the dispersion of light is neglected : it is explained 
in §§ 37, 38. 



-K>a^- 



Fig. 11. — Reflection by a Concav 
Mirror. 




Fist. 12. — Refraction. 



28. Refraction by a Prism. — A ray of light passing from one 
medium, as air, into another of a different density, as glass, is bent 
out of its course, unless it strikes the surface of the second medium 

perpendicularly. This bend- 
ing is called refraction. 

The ray AB, striking ob- 
liquely on the surface XY of 
the glass prism XYZ, is re- 
fracted and travels along BC ; 
at emergence, the ray is again 
refracted, taking the direction 
CD. 

29. Action of a Number 

of Prisms. — By inspection of 

Fig. 13 we see that a number of pieces of glass may be so arranged 

that a system of parallel rays, in the plane of the paper, falling upon 




Refraction by Prisms. 



THE TELESCOPE. 



21 



them will be converged to a common point. If the number of 
pieces of glass be largely increased, each piece being very small, the 
broken lines ABC and ADC will approach closely to arcs of circles. 
Hence, if a single piece of glass be so shaped that ABC and ADC 
will be nearly arcs of circles, the system of rays will be converged, 
as above, to a single point, called the focus. 

30. Lenses. — A common burning glass is circular in form, and 
when looked at edgewise has the shape of (<?) in Fig. 14. The two 
surfaces of the glass are portions of spherical surfaces. The sun's 



(b) 





Fig. 14. — Lenses. 



Fig. 15. — Rays made Divergent. 



rays striking on the glass are converged to a focus. Such a glass 
is called a double convex lens : (b), one side of which is flat, is a 
plano-convex lens; (c) is double concave; (d) is plano-concave; 
(c) and (d), instead of bringing a system of parallel rays to a focus, 
make them diverge as shown in Fig. 15. 




Fig. 16. — Visual Angle. 



31. Visual Angle. — The visual angle of an object is the angle 
made by two lines drawn from the eye to the extremities of the 



22 



DESCRIPTIVE ASTRONOMY. 



object. In Fig. 16, the object AB is placed in three different 
positions AB, A' B', and A" B" ', the eye being at E. The visual 
angles are respectively AEB, A! E B', and A" E B" '. The nearer 
the object is to the eye, the greater is the visual angle, and the 
larger the object appears. If the object be carried away from the 
eye, the visual angle will become less, and the object will appear 
smaller. 




Object and Image. 



32. Formation of an Image. — The action of a double convex 
lens in forming an image of an object may be easily seen in a 
photographic camera. Let the arrow in Fig. 17 represent a tree 
which is to be photographed. From the point of the arrow come 
innumerable rays of light, which strike the outer face of the lens, 
and are refracted by it to a common focus at A. Similarly the 
rays from the other end of the arrow are brought to a focus at B. 
When the photographer adjusts his ground-glass so that the points 
A and B lie on its surface, an image of each of these points is 
formed on the glass. Every other point of the arrow images itself 
on the glass, in like manner. The object-glass of a telescope is the 
large lens at the end farthest from the eye : its function is, like the 
photographer's lens, to form an image of the object viewed. 

33. Action of the Eyepiece. — In a telescope there is no ground- 
glass to receive the image formed by the object-glass, but the rays 
of light pass on through another lens, called the eyepiece, before 
reaching the eye. The three rays shown in Fig. 18 as diverging 
from the point A in the image are rendered more nearly parallel 



THE TELESCOPE. 



23 



to each other in passing through the eyepiece, and are sharply 
bent. The rays diverging from B are bent in the same fashion. 
Prolong the central one of the rays coming from A backward to 
any convenient point, Y, and the central ray from B to X, and draw 
the arrow XY. AB seen 



the 



eyepiece 




x 



through 

looks as large as XY 
would without the eye- 
piece, XEY being the 
common visual angle. 

34. A Simple Refract- 
ing Telescope. — Placing 
the object-glass and eye- 
piece at opposite ends 
of a tube, we have a 

telescope. In Fig. 19, suppose the telescope to be pointed at a 
distant tree : the rays A', A, A" ', coming from the top of the tree, 
meet at the focus a, and, passing on through the eyepiece, enter the 
observer's eye. The rays B', B, B", coming from the bottom of the 



Y 



Fig. 18. — Lens as Eyepiece. 




Fig. 19. — Object-glass and Eyepiece. 

tree, meet at the focus b, and pass on through the eyepiece to enter 
the observer's eye. The central rays, A and B, of the two systems 
meet at E, and to the observer the tree appears to subtend a visual 
angle b'E a'. If the object-glass be now removed, and the observer, 
placing his eye at O, where the centre of the object-glass was, looks 
at the tree, it will subtend a visual angle of AOB. If he were to 
stand near the eyepiece, the visual angle would be a trifle smaller 
than AOB, since he is a little farther from the tree. But when 
he looked through the telescope, the tree appeared to subtend the 



24 DESCRIPTIVE ASTRONOMY. 

angle b'E a', which is much larger than AOB. This explains why 
a telescope magnifies an object. 

35. Object-glasses of Various Sizes. — When one looks at a star, only 
those rays which fall upon the pupil enter the eye. Were the pupil 
larger, more rays would enter, and the star would appear more 
brilliant. If the area of the object-glass of a telescope is one 
hundred times the area of the pupil of the eye, one hundred times 
as much light will fall upon it as upon the unaided eye. The pupil 
of a human eye, when not exposed to a bright light, has on the 
average a diameter of one fifth of an inch. An object-glass one inch 
in aperture (as its diameter is called) has a diameter five times as 
great as that of the pupil of the eye. A twenty-inch object-glass 
has a diameter one hundred times as great. Geometry teaches that 
the areas of two circles are to each other as the squares of their 
diameters : hence a one-inch object-glass collects not merely five 
times as much light as the pupil of the eye, but 5 2 , or 25 times as 
much. A twenty-inch "objective" collects iOO 2 , or 10,000 times as 
much light from any star, as the unassisted eye does. About 
18 per cent of this light is lost in passing through the object-glass 
and eyepiece. 

36. Magnifying Power of Eyepieces. — After the object-glass has 
made a brilliant image of an object, at the focus, the eyepiece 
magnifies the image, as shown in § 34. By using lenses of dif- 
ferent degrees of curvature, different "magnifying powers" are 
obtained. 

If the apparent diameter of the planet Jupiter, for instance, were 
increased sixteen fold by a telescope armed with a certain eyepiece, 
the magnifying power of that eyepiece would be sixteen diameters. 
Very high magnifying powers cannot be used advantageously, be- 
cause of the disturbances continually going on in our atmosphere. 
The rays of light from a star, coming through various disturbed 
strata of air of different densities, are bent hither and thither, so that 
the image of the star in the telescope dances about, and looks 
blurred. The higher the magnifying power employed, the worse 
the blur. Ordinarily, an eyepiece the magnifying power of which 
is more than twenty times the aperture of the object-glass (in inches), 
cannot be used to advantage. But when the atmosphere is exceed- 
ingly calm, a power of one hundred times the aperture may be used 



THE TELESCOPE. 



2 5 



on a bright object. A glass six inches in aperture would then bear 
a power of six hundred diameters. 

37. Dispersion of Light. — When a beam of sunlight is passed 
through a prism, and then allowed to fall on a screen, a colored 
spot is seen where the light strikes the screen. The spot is red at 




Fig. 20. — Dispersion 



one end and violet at the other. Careful experiments show that 
sunlight, when passed through a prism, is decomposed into the 
following colors : red, orange, yellow, green, cyan-blue, ultra- 
marine blue, and violet. The light thus decom- 
posed is said to be dispersed. The figure shows 
that the red rays are not refracted (deviated from 
their original direction) as much as the violet rays. 
A prism of flint glass separates the red rays from 
the violet more widely than a prism of crown 
glass. 

38. Correction of Dispersion. — The dispersion 
caused by one prism may be counteracted by- 
passing the dispersed beam through a similar in- 
verted prism. The final emergent beam is parallel 
to the original beam. By passing a beam through 
two prisms of different angles, one being of crown 
glass, and the other of flint, it is possible to correct the dispersion 
very nearly, and at the same time to alter the direction of the 
beam. 




Fig. 21. — Dispersion 
Corrected. 



26 



DESCRIPTIVE ASTRONOMY 



39. An Achromatic Object-glass. — This is shown in Fig. 22. Al- 
most all object-glasses are made of two lenses : the outer lens is of 
crown glass, and is double convex ; the inner is of flint glass, and is 




2. — Achromatic Object-glass. 



nearly plano-concave. Such an object-glass is said to be achromatic 
(without color). The largest and finest object-glasses in the world 

have been ground by the firm 
of Alvan Clark and Sons of 
Cambridge, Mass. In 1881, 
Prof. Abbe and Dr. Schott, of 
Jena, Germany, began a series 
of experiments which have re- 
sulted in the manufacture of 
lenses of various refractive and 
dispersive powers, combina- 
tions of which give almost per- 
fect achromatism. Some of 
these lenses, however, tarnish 
in time. 1 

40. Achromatic Eyepieces. — 
Eyepieces are made achro- 
matic also, for a bad eyepiece 
undoes the work of a good 
object-glass. There are two 
common forms, the Huygheni- 
an, or negative, and the Rams- 
den, or positive : the former is more achromatic than the latter. 
The large lens of the negative eyepiece receives the rays from the 

1 Mr. J. A. Brashear, the well known optician, of Allegheny, Pa., makes a specialty 
of grinding lenses of the new glass. 




Fig. 



Alvan Clark. 



THE TELESCOPE. 



27 



object-glass just before they come to a focus; the focus is formed 
between the two lenses of the eyepiece. In the case of the positive 



NEGATIVE 



POSITIVE 



Fig. 24. — Eyepieces. 



eyepiece, the rays come to a focus before they reach the eyepiece. 
A positive eyepiece can be used as a hand magnifying glass, but a 
negative cannot. 

41. The Reflector. — A reflector, or reflecting telescope, receives 
its name from the fact that, instead of an object-glass, it has a large 
concave mirror, which reflects to a focus the rays from a celestial 




EYEPIECE 
Fig. 25. — Path of Rays of Light in a Reflector. 

object. Reflectors have many forms, differing in minor particulars. 
The form most used is the Newtonian, devised by Sir Isaac Newton, 
which is shown in Fig. 26. 

The mirror is now made of glass, on which a thin film of silver is 
deposited by a chemical process. 

42. Comparison of Refractors and Reflectors. — The silver makes a 
brilliant reflecting surface, and there is no trouble from dispersion 
of light, as in the case of a refractor. This gives a reflector an 
advantage for photographic and spectroscopic work. But a large 
reflector has many disadvantages when compared with a refractor 
of equal power. The silver film tarnishes, and must be renewed 
periodically. Slight deformations of the concave surface, caused by 



28 



DESCRIPTIVE ASTRONOMY. 




Fig. 26. — A Newtonian Reflector made ey Brashear. 



THE TELESCOPE, 



2 9 




' ' '///( •'" ' ',VA'. '••■//-,- ' .', ,J '" ■ ' ■' 



Fig. 27. — Lord Rosse's Six-foot Reflector 

(From Scribner's Magazine, by permission.) 



30 



DESCRIPTIVE ASTRONOMY. 



a difference in temperature between the front and the back of the 
mirror, or by sagging because of its own weight, distort the image 
of the object looked at. 

43. Some Noted Reflectors. — Lord Rosse has at Parsonstown, 
Ireland, the largest reflector ever built. The mirror, which con- 
sists of an alloy of copper and tin, is six feet in diameter. Mr. 
A. A. Common, a wealthy English amateur, has made a silver on 
glass reflector, sixty-two inches in aperture, which is more effi- 
cient than any other ever constructed. Such an instrument is 
capable of marvellous work in photographing faint objects like 
nebulae. Fig. 27 represents Lord Rosse's six-foot reflector. There 
is no reflector in America which approaches these in power. 

44. An Equatorial Mounting. — Though a large telescope be well- 
nigh perfect optically, it will be practically useless unless well 






w 

H 




c 


• 




\r 



Fig. 28. — Scheme of an Equatorial Mounting. 



mounted. On account of the diurnal motion of the stars, if the 
telescope is pointed at one, and is motionless, the star will quickly 
pass out of the field of view. The best mounting, when an object is 
to be watched for some time, is therefore one which will enable the 
telescope to follow the object most easily. It has been shown (§17) 
that the heavens appear to rotate daily about an axis drawn from 
the north celestial pole through the observer. Let a strong steel 
axis, AB, be supported at each end on pivots A and B, on which it 
may turn, and let it point to the north celestial pole. This steel 
axis may then be considered as a portion of the axis about which 
the celestial sphere appears to rotate. Through the hole O let 



THE TELESCOPE. 3 1 

the axis CD, to which the telescope is fastened, be thrust, and 
the weight W be placed at C, to balance the telescope. Let the 
telescope be pointed to any star, and the axis AB be turned by 
suitable clockwork, so that it will make one revolution in twenty- 
four hours. Then the telescope will move just as it would if it were 
actually fastened to the axis of the celestial sphere, and will con- 
tinue to point to the star. 

45. Illustration of the Principle of an Equatorial Mounting. — To 
get a clearer idea of this, one may imagine that the earth's axis is a 
wooden pole running through it; further, that a person in the interior 
of the earth nails a lath to this pole in such a way that the lath 
points to the city of Boston. The lath will continue to point to 
Boston as the earth rotates. If the lath had originally pointed to 
any other city, it would continue to do so, whether the city were 
near the equator or in the vicinity of one of the poles. The lath 
represents the telescope, and the city, a star. 

46. The German Form of an Equatorial Mounting. — The form of 
mounting just described is not suited to 

a large refractor, because the axis AB 
would necessarily be very large and 
cumbrous. In Fig. 29, AB is parallel 
to the earth's axis, and is called the 
polar axis. CD, the declination axis 
(§122), runs through the " sleeve " 
EF, which is bolted to the top of the 
polar axis. The telescope tube is fas- 
tened to the declination axis. W is a 

counterpoise, to balance the weight of Fig. 29. — A German Equatorial. 

the telescope, so that the whole mechan- 
ism will be nicely poised on the polar axis. The German form 
of mounting is shown in Fig. 30. 

47. Management of a Telescope. — The following directions are for 
inexperienced observers using small telescopes. 

1. Do not look through a closed window; the irregularities of 
the pane of glass will distort objects. 

2. Do not point the telescope out of an open window, in a room 
warmer than the external air; the warm air currents rushing from 
the room cause the images of objects to waver. 







32 



DESCRIPTIVE ASTRONOMY. 




Fig. 30. — The Lick Telescope. 



THE TELESCOPE. 33 

3. If a telescope has several eyepieces, use the one of lowest 
power first; if the object appears distinct, you may try a higher 
power. 

4. For comets and nebulae use low powers. 

5. In general, avoid touching the telescope when looking 
through ; you may shake it. 

6. Focus carefully, by sliding the eyepiece in or out, until the 
view is most distinct. Different eyes frequently require different 
adjustments of the eyepiece. 

7. Clean the object-glass rarely, and carefully. A little dust on 
it will be of no appreciable detriment. Dust may be removed with 
a camel's-hair brush. Never rub the glass with any material. 

8. Keep the eyepieces clean. 

EXERCISES. 

48. 1. If an incident ray be perpendicular to the surface of a 
plane mirror, what direction will the reflected ray take? 

2. Is the image on the ground-glass of a photographer's camera 
upright or inverted? Does an object which is at the right of 
another, as seen with the naked eye, appear at the right of it on the 
ground-glass? 

3. If you look through the wrong end of a telescope, are objects 
magnified, or are they minified? 

4. The object-glass of the Lick telescope is 36 inches in diameter. 
If the diameter of the pupil of one's eye be one fifth of an inch, how 
many times as much light as the unaided eye does the Lick glass 
receive from a star? 

5. When looking across a landscape at a distant fixed object, 
did you ever notice that the object trembled slightly, or had a wavy 
appearance? 

Explain the cause of this appearance. 

6. Draw a circle having a radius of two inches. With the same 
centre draw another having a radius of i\ inches. At some point 
of the inner circle draw a line about three inches long, tangent to 
it. From the same point draw outward a perpendicular to the tan- 
gent, and two oblique lines, making respectively angles of 45° and 
io° with the tangent. The smaller circle represents the earth, and 



34 DESCRIPTIVE ASTRONOMY. 

the space between the circles the atmosphere. The tangent is the 
observer's horizon. When a man looks straight up at the heavens, 
does he look through more atmosphere, or less, than when he looks 
nearly horizontally? 

7. On a given night does the moon appear more brilliant when 
in mid-heaven than when rising or setting? Give a reason for your 
reply. 

8. On a clear, moonless night, do stars near the zenith twinkle 
more violently, or less, than those near the horizon? Why? 

9. Did you ever see a fixed star set? Would the observation of 
its setting be difficult at sea? 

10. Which rays are the more refrangible, the red or the violet? 

11. When a person is looking through a telescope, if you 
hold your finger in front of the object-glass and near it, will he 
see it ? 

12. If a person were looking through a telescope at the full 
moon, and another suddenly covered up one half of the object-glass, 
how would the appearance of the moon be changed? 

13. In an equatorial mounting, what angle does the declination 
axis make with the sight line? What angle does the declination 
axis make with the polar axis ? 

14. If one takes hold of the telescope at the eye end and moves 
it about its axis into various positions, will the angle between the 
declination axis and the sight line change? Will the angle between 
the declination axis and the polar axis change? 

15. An astronomer using an equatorial (as the whole instrument 
is usually designated) in a fixed observatory points the telescope to 
various parts of the sky. 

{a) Does the polar axis turn? 

(b) Does the polar axis point to different points on the celestial 
sphere? 

(<:) Does the declination axis point toward different points on 
the celestial sphere? 

16. (a) Does the declination axis ever lie in a plane perpen- 
dicular to the polar axis? 

(b) Does the declination axis always lie in a plane perpendicular 
to the polar axis? 

(c) If the polar axis be turned through one revolution, what 



THE TELESCOPE. 35 

circle would the declination axis, prolonged to the celestial sphere, 
trace on it? 

REMARK. — In answering the following exercises, scholars may 
obtain assistance by using a pair of shears. Let the cutting edge 
of one blade represent the sight line, the edge of the other the polar 
axis, and the rivet holding the blades together the declination axis. 
The shears should be so held in the hand that the blade representing 
the polar axis points to the north celestial pole. 

17. If the sight line of a telescope equatorially mounted be 
placed (by turning the telescope about the declination axis) perpen- 
dicular to the polar axis (see Fig. 28), it will be parallel to the 
plane of a well known circle. What is the name of the circle? 

18. When the sight line of an equatorial telescope has been 
placed perpendicular to the polar axis, if the latter be rotated by 
the clockwork, what circle will the sight line, prolonged to the celes- 
tial sphere, trace upon it? 

19. If the sight line of an equatorial be placed parallel to the 
polar axis, toward what point on the celestial sphere will the tele- 
scope point? 

20. The telescope, placed as in the preceding exercise, is rotated 
on its polar axis. 

(a) Will the sight line continue to be parallel to the polar axis? 

(J?) What sort of a geometrical figure will the sight line, pro- 
longed, trace on the celestial sphere? 

(c) Would that figure, if it could be seen from the earth, appear 
large or small ? 

21. If the sight line of an equatorial be placed at an oblique 
angle with the polar axis, and the instrument be rotated on that 
axis, will the sight line describe a circle on the celestial sphere? 

22. Do stars near the celestial equator seem to move across the 
sky more swiftly than those near the pole, or more slowly? Will 
the clockwork of an equatorial (when so rated that a star near the 
celestial equator will be kept in the field of the telescope) need a 
special adjustment, to enable it to keep a star near the pole in the 
field? 



36 DESCRIPTIVE ASTRONOMY. 



CHAPTER IV. 

THE SUN. 

" But yonder comes the powerful king of day, 
Rejoicing in the east. The lessening cloud, 
The kindling azure, and the mountain's brow, 
Illumed with fluid gold, his near approach 
Betoken glad." 

Thomson. 

49. Distance and Diameter. — The average distance of the earth 
from the sun is 92,900000 miles. 

Prof. Mendenhall has said that, if a babe could instantaneously 
reach across this stupendous gulf and touch the glowing surface of 
the sun, he would never realize that his hand was burned, for, though 
the nerves transmit sensations to the brain with great rapidity, over 
one hundred years would be required for this message. 

The sun's diameter is 866,500 miles, 109.5 times as great as that 
of the earth. Were the earth to swell to the size of the sun, and were 
men to increase in the same ratio, an average man would be 625 feet 
tall. Since he would also be 109.5 times as broad and 109.5 times as 
thick as at present, his bulk would become 109. 5 X 109. 5 X 109. 5, or 
more than 1,300000 times as great as now. 

The force of gravity at the sun's surface being 27.6 times as 
powerful as at the surface of the earth, our giant, if transported to 
the sun, would weigh 2,750000 tons, if of ordinary build. 

50. How to View the Sun through a Small Telescope. — The sun 
may be viewed, for a moment, through a pinhole in a card, without 
injury to the eye, but the observation is not to be recommended. 

A small telescope may be directed toward the sun by moving it 
until its shadow, thrown on a book held near the lower end, is as 
small as it can be made, and a dazzling light issues from the eye- 
piece. A dark shade glass may then be held close to the eyepiece, 
and one may look through. Great care should be taken to avoid 
getting the full blaze of sunlight into the eye, and one should not 



THE SUN. 



37 



continue looking through a dark glass, if the sun is uncomfortably 
bright. 

The dark glass and eyepiece soon become hot, and may break if 
the telescope is kept pointed at the sun too long. The amount of 
light and heat may be much diminished by covering the object-glass 
with a piece of paper having a circle an inch or more in diameter 
cut in it, but one will not be able to see the more delicate details of 
the sun's surface quite as well. 

51. Use of a Screen. — If a piece of white paper be held about a 
foot from the eyepiece, and the eyepiece pulled out a fraction of an 
inch beyond its proper focal position, an image of the sun will be 
formed on the screen. By careful adjustment of the eyepiece, the 
sun spots will usually be well seen. 





Fig. 31 



The Sun's Image on a Screen 



Fig. 32. — Absorption of Light 
by the Sun's Atmosphere. 



Another screen fastened to the telescope, as shown in Fig. 31, 
so as to throw its shadow on the first screen, will improve the view. 
Several persons can thus view the sun at once. The telescope, unless 
equatorially mounted and driven by clockwork, must be moved 
about every minute in order to keep the sun's image on the screen. 
If the eyepiece be of high magnifying power, the entire sun cannot 
be seen at once. Special eyepieces are made for solar work, which 
obviate, in large measure, the inconveniences rising from its intense 
light and heat. 

52. The Photosphere. — The name photosphere (sphere of light) 
is given to the brilliant surface of the sun. This surface, as seen 
through a telescope, has a grayish cast. It is brighter at the centre 
than near the edge. The cause of this is shown in Fig. 32. To an 



38 DESCRIPTIVE ASTRONOMY. 

observer situated at the right of the figure, rays coming from A or 
B, which would be at the edge of the sun, pass through more of the 
sun's atmosphere than those coming from C ; thus they suffer a 
greater absorption, and appear fainter. The photosphere corre- 
sponds to the crust of the earth, but it is far from solid : it is to be 
regarded as a cloud-like shell of intensely heated vapors. As water 



* 



1 4 



Fig- 33- — Facuue Observed Visually. 

on the earth, when evaporated, rises and condenses in the upper air 
into clouds, so the vapors of metals, rising from the interior of the 
sun, condense into drops and form the photospheric clouds. 

53. Rice Grains. — The photosphere, when viewed under favorable 
conditions, is seen to be mottled with small bright objects, which 
have been likened to rice grains floating in a plate of soup, or snow- 



THE SUN. 39 

flakes on a gray cloth. To an observer situated a few thousand miles 
from the earth, the cloud forms on a cloudy day might exhibit much 
the same appearance. The cloud formation which navigators call a 
mackerel sky is suggestive of it. 

54. Faculse. — Faculae (from the Latin word f acuta, a small torch) 
are shown in Fig. 33. They are best seen near the limb 1 of the sun, 
and are especially abundant in the neighborhood of spots. The 
photosphere is agitated by such furious storms that its outer surface 
rises in mountain-like ridges, or crests, like the waves of a raging 
sea ; these, projecting upward through the lower part of the solar 
atmosphere, look brighter than the general background. 

A man standing on the summit of Pike's Peak on a clear night 
would see the moon through much less of our light-absorbing at- 
mosphere than if he were at sea level. Hence the moon would appear 
more glorious. 

In like manner, an observer on the moon could see high terrestrial 
mountains better than the low level of the plains. The faculae are 
more distinct near the limb of the sun, because the general back- 
ground is darker there. Recent photographs, however, show them 
well near the centre of the sun's disk. They are sometimes 20,000 
miles long, and more than 200 miles high. The faculae seem to form 
an irregular network over the entire surface of the sun. 

SUN SPOTS. 

55. General Appearance of a Spot. — Sun spots reside in the photo- 
sphere. In looking at the sun with a telescope, specks of dust on 
the eyepiece are seen as black spots on its face. But a true sun spot 
is distinguished from these by the fact that it has a dark central 
portion surrounded by a lighter border. The dark central part is 
called the umbra ; the border is the penumbra. Some portions of 
the umbra are frequently darker than others. The impression given 
to an observer is that these dark places are deep holes. The 
penumbra is composed of filaments which point inward toward the 
umbra. Sometimes bridges of light cross the umbra from side to 
side, or, if too short, seem to project out over it, as a fishing rod 
hangs over a pool. 

1 The word " limb " is used by astronomers to denote the edge of the disk. 



4 o 



DESCRIPTIVE ASTRONOMY. 



Occasionally, when a spot is just on the edge of the sun, it is 
seen as a notch in its smooth periphery. Hence spots are thought 
to be saucer-like depressions below the general level. 





Fig. 34. — Sun Spot. 

(From Langley's " New Astronomy," by permission.) 



56. Changes in Appearance. — These objects are not of fixed form, 
like mountains or lakes, but change continually. The change in a 
day is usually very marked. Bridges may form or disappear ; the 
spot may grow or diminish very perceptibly, may break into two or 
more spots, or may even vanish altogether. The changes are on 
some occasions so rapid as to make it impossible to sketch the spot. 
An area as large as the United States may vanish in a quarter of an 
hour. The lifetime of a spot averages three or four weeks ; one has 
been known to last a year and a half. 

57. Dimensions. — The smallest ones observed with large telescopes 
have umbrae 500 miles in diameter. In a large spot the diameter of 
the umbra may reach 50,000 miles. The largest group in recent 



THE SUN. 41 

years was visible during Feb. 5-17, 1892. It was 150,000 miles long 
and 75,000 miles broad, the central spot of the group being 100,000 
miles long, and half as broad. Such a group may be seen without 
a telescope, by looking through a colored or smoked glass, or even 
through a bank of haze when the sun is near the horizon. 







' *£ lL -^ 










Wk 




- v3F- 




--— IJPVv- 


v 


*#|^p 




i 



Fig' 35- — Large Sun Spot. 

58. Movements : Rotation of the Sun. — If a spot be seen at noon of 
a certain day in the centre of the sun's disk, at the next noonday it 
will appear at the right of its former position. In a week it will 
have passed the western limb, and in two weeks thereafter it will 
emerge into view on the eastern limb, if still in existence. This 
shows that the sun rotates on an axis, as the earth does. Care- 
ful observations made on spots show that different portions of the 
sun rotate in different times. Near the solar equator spots make 
their circuit in twenty-five days, but spots situated half way from 
the equator to the poles consume twenty-seven days in a like 
journey. 

Spots are rarely seen at the solar equator, and never more than 
half way to the poles. No one has yet explained satisfactorily this 



42 DESCRIPTIVE ASTRONOMY. 

distribution of the spots, or the irregularity of the rotation time of 
different portions of the sun's surface. 

59. Periodicity. — In 1826, Schwabe, a magistrate in the little 
German town of Dessau, began for his own pleasure to count the 
number of sun spots visible in his telescope each day. After twenty- 
five years of patient endeavor he found that he had been, like Saul, 



t*m 






Ski 



C-.M* 



Fig. 36. — The Sun at the Time of a Spot Maximum. 

" going out to seek his father's asses, and finding a kingdom." For 
he discovered that spots were much more numerous in some years 
than in others, and that the numbers changed with considerable regu- 
larity. Later investigations have fixed the average period as being 
I I.I years. A maximum of spottedness occurred in 1893, but was 



THE SUN. 



43 



not very pronounced. After that the spot activity gradually lessened, 
and is expected to be feeblest about 1900. Then for weeks at a time 
no spot may be visible ; after the minimum, the spotted area will 
increase until about 1905, when another maximum is due; at a time 
of maximum the sun is never free from spots. Times of maxima 
and minima may vary a year or two from those predicted. The 
cause of this periodicity is unknown ; it has been surmised to be due 
in some way to planetary influences. 

60. Observations by Carrington and Hodgson. — Very violent dis- 
turbances are at times noted in the neighborhood of spots. The 




Fig- 37- — Photographs of the Disturbance of July 15, 1S92. 



classic observation of Carrington and Hodgson, two English 
observers, was made on Sept. 1, 1859. Near the edge of a great 
spot there suddenly appeared two luminous masses, the length of 
each of which was equal to the earth's diameter. So dazzling 
were they that they were estimated to be five times as brilliant as 
the general surface of the sun. They moved side by side across the 



44 



DESCRIPTIVE ASTRONOMY. 



spot with a velocity of over ioo miles a second, growing fainter; in 
five minutes they had faded from view. These were probably the 
product of an eruption of marvellous energy. 

61. Disturbance on July 15, 1892. — On this date Prof. George 
E. Hale 1 took a photograph of a large spot which had two 
umbrae separated by a bright bridge of light. Another photo- 
graph, taken twelve minutes after, showed an exceedingly bright 
object, shaped somewhat like a fish-hook, the hook end being 
baited with a brilliant ball which was near the centre of the 
umbra. In half an hour thereafter, the region of the spot was 
completely covered with brilliant outbursts, so that the umbrae 
were no longer visible in the photograph. Two hours later, 
the disturbance, which extended over an area of four billion 

square miles, had disappeared 
entirely. It seems to have been 
high above the spots, which were 
unchanged by these terrific out- 
bursts. 

62. Cyclonic Motion. — On rare 
occasions a spot is found which 
exhibits a motion of rotation ; 
sometimes an entire revolution 
is accomplished in a few days, 
but usually only a portion of a 
revolution is accomplished. In 
such spots the filaments of the 
penumbra are curved, as shown 
in Fig. 38. The motion of these 
spots is analogous to that of 
whirlwinds and cyclones upon the earth. But the analogy must 
not be pressed too far, for terrestrial cyclones in the northern 
hemisphere always rotate in a left-handed direction (opposite to 
that of the hands of a watch). Sun spots have no regularity of 
rotation. 

63. Nature of Sun Spots. — Upon this there has been much specu- 
lation. No theory has yet been found which accounts for the 




Fig. 38. — Cyclonic Motion in a Spot. 



1 Director of the Yerkes Observatory. 



THE SUN. 45 

observed appearances fully. Prof. Young's * theory is given in 
substance below. 

When the fiery gases imprisoned beneath the photospheric cloud- 
shell burst forth at any weak place in the shell, there is a temporary 
diminution of the upward pressure against the photosphere in that 
locality. Hence the photosphere sinks somewhere in the neighbor- 
hood, an irregular shallow cavity being formed. The materials 
thrown out by the eruption are cooled in the upper regions of the 
sun's atmosphere, and fall back into the cavity. The light from 
below, struggling up through this mass of comparatively cool vapors, 
is dimmed by absorption. Hence the umbra, though really intensely 
luminous, sends to us less light than the surrounding photosphere, 
and looks black by contrast with it. 

The filaments of the penumbra are supposed to be long drawn 
out rice grains (§ 53). 

64. Sun Spots as Causes of Changes of the Weather, etc. — Many 
have been the attempts to show that the maxima and minima of 
spots affect the meteorological conditions. One investigator dis- 
covers that years when sun spots are at a maximum are more 
rainy than the average, and that cyclones and other violent storms 
are then most prevalent. Another concludes that such years are 
hotter than the average, while a third finds them to be cooler. 
Others attribute the recurrence of Asiatic cholera, variations in the 
amount of atmospheric ozone, or the prevalence of commercial panics, 
to the direful spots. The data on which these conclusions are based 
are, in general, so conflicting as to produce, in one who examines 
them, much weariness of the flesh and little satisfaction of the spirit. 

65. Magnetic Storms. — A compass needle does not always point 
in the same direction. One of the large and accurate ones used in 
magnetic observatories shifts in direction a few minutes of arc 
every day, vibrating to and fro. Sometimes these oscillations are 
greatly increased, and are subject to no perceptible law ; the needles 
seem fairly beside themselves with magnetic excitement. 

Powerful currents traverse the telegraph wires, and send mes- 
sages in an unknown tongue ; private lines are temporarily worked 
in the nervous systems of the operators ; the regular electrical 

1 C. A. Young, Professor of Astronomy in Princeton University, one of the most 
distinguished students of the sun. 



46 DESCRIPTIVE ASTRONOMY. 

apparatus is set on fire at times. At night the weird auroral beams 
execute their most fantastic dances. 

66. Connection of these Storms with Solar Outbursts. — The singular 
event mentioned in § 60 took place during a great magnetic storm, 
which was raging upon the earth. In Washington and Philadelphia 
the telegraph operators were severely shocked. At Boston a flame 
of fire followed the pen of a recording telegraphic instrument. 

There were fine auroral displays in all parts of the world ; even 
countries near the equator enjoyed the spectacle, to them almost 
unknown. During the years 1873 to 1892 there were three especially 
severe magnetic storms on the earth. There were also three very 
notable displays of sun spots. The magnetic storms occurred at 
the times of the greatest development of the spots. 

67. The Storm of February, 1892. The great spot of Feb. 5-17, 
1892 (§ 57), was accompanied by a magnetic storm which raged 
on Feb. 13 and 14, when the spot group had attained its max- 
imum dimensions, covering -^50 of the sun's visible hemisphere. 
Fine auroras flashed out during this storm. Magnetic recording 
instruments were more violently disturbed than for ten years pre- 
viously. An earth current awakened a sleeping operator, in France, 
by ringing his signal bell. Nearly a month afterwards, when the 
spot, much enfeebled, came by reason of the sun's rotation into the 
same apparent position on its disk, another bright aurora accom- 
panied by a magnetic storm occurred. 

68. Frequency of Magnetic Storms. — An examination of the 
records of these storms shows that they too have times of maxi- 
mum and minimum, and that these times correspond closely with 
those for sun spots. That there is some connection between the 
two is no longer doubtful, though the most distinguished physicists 
are unable to explain the nature of the relation. Conspicuous sun 
spots, or other solar disturbances, are not always accompanied by 
magnetic storms on the earth. This is not astonishing, however ; 
for terrestrial storms often occur in which there is no special dis- 
play of electrical phenomena. 

Some are of the opinion that, when a solar storm is associated 
with electrical disturbance there, the disturbance is propagated with 
the speed of light through the ether to the earth, which is thrilled 
responsively. 



THE SUN. 



47 



THE SPECTROSCOPE. 



69. Description of the Instrument. — We learned in § 37, that 
white light might be resolved into its component colors by pass- 




Fig. 39. — A Spectroscope (made by Brashear). 

ing it through a prism. The peculiarities of light thus dispersed 
are conveniently studied by means of the spectroscope ; the action 




Fig. 40. — Plan of a Spectroscope. 

of a simple form of this instrument is shown by Fig. 40. At the 
point S is a slit, shown in Fig. 41. It is a straight 




48 DESCRIPTIVE ASTRONOMY. 

between two pieces of metal, x and jy, shown in the cut ; x is movable 
by the screw a, so that the width of the slit may 
be altered at pleasure. S is put at such a dis- 
tance from the lens A that the rays of light 
coming from S are rendered parallel by pass- 
ing through A. These rays then strike the 

Fi?. 41. — Slit of a Spec- . r . , ,i , 1 

troscope prism, are refracted by it, enter the telescope, 

and come through to the eye at E. 

70. Slit Illuminated by Red Light. — Suppose that in front of the 
slit we could burn some substance which gave out a red light, 110 
other color except a particular shade of red being given out by the 
substance. Let the slit be almost closed. On looking through the 
telescope one would see a fine red line, just the shape of the slit. 
If half the slit were covered by a card, the observer would see a 
line only half as long as before ; if the slit were widened by turning 
the screw a (Fig. 41), the image seen by the observer would be 
widened likewise. If a small circular hole were put in place of the 
slit, the observer would see through the telescope a red circle. 

71. Slit Illuminated by Lights of Different Colors. — The flame of 
an alcohol lamp is almost colorless. Place on the wick some 
common salt and the flame will be colored yellow, this hue being 
due to sodium. Put the yellow flame in front of the slit, and the 
observer will see a yellow line, the image of the slit. Try a similar 
experiment with a salt of thallium, and a green slit image will be 
seen. Next lay on the wick of the lamp both common salt and a 
salt of thallium. Both yellow and green light will enter the slit, but 
in passing through the prism the yellow rays will not be bent out of 
their course as much as the green rays. Therefore, if the slit be 
nearly closed, the observer will see two fine lines, one yellow and 
the other green, standing side by side. If any number of colors be 
admitted at once, there will be the same number of slit images 
standing side by side. When a candle, which gives a light com- 
posed of a great number of tints, illuminates the slit, the images 
are so closely crowded together that they form a continuous band 
of color from red to violet (§ 37). This is called a continuous spec- 
trum. Were the candle capable of giving out all colors but green, 
there would be in the ribbon of light or spectrum a dark gap be- 
tween cvan-blue and vellow. 



THE SUN. 



49 



72. White Light shining through an Incandescent Gas. — Arrange 
the apparatus as shown in Fig. 42. Let the sodium and thal- 
lium be giving in the spectroscope their yellow and green lines. 
Lift up the screen so that the calcium light 1 shines through the 
glowing gases in the flame of the spirit lamp into the instrument. 




Production of Spectra. 



Instantly, the spectrum will change to a many-colored ribbon, like 
that caused by a candle, except that, where the bright lines due to 
sodium and thallium formerly were, the spectrum will be crossed 
by dark lines. Such a spectrum is called an absorption spectrum. 
The two spectra are shown without the colors in Fig. 43. Put the 



GREEN YELLOW 




THALLIUM SOD-IUM 

LINE LINES 



1 



BRIGHT LINE 
SPECTRA 



DARK LINE 
SPECTRA 



Fig. 43. — Spectra. 

screen in place again, so as to cut off the rays from the calcium 
light, and the bright lines will reappear. Remove the alcohol lamp 
and the screen, and the calcium light will produce a continuous 
spectrum. 



1 The calcium light is produced by introducing a piece of lime into a flame caused 
by burning oxygen and hydrogen gases together. Such a light has been seen over one 
hundred miles in full daylight. 

4 



50 



DESCRIPTIVE ASTRONOMY. 



73. Laws of Spectrum Analysis. — By an exhaustive series of experi- 
ments similar to the preceding, the following laws have been 
discovered. They are called Kirchhoff's laws. 

I. An incandescent solid, or liquid, or even gas under high 
pressure, gives a continuous spectrum. 

A candle or kerosene lamp gives a continuous spectrum because 
nearly all of its light comes from glowing particles of solid carbon 

(which when cooled form soot). 

2. A glowing gas, unless con- 
densed by high pressure, gives a 
discontinuous spectrum made up of 
bright lines or bands. 

The spectrum of iron vapor con- 
sists of hundreds of bright lines. 
Sodium vapor gives a small number 
of lines, the most conspicuous of 
which has been mentioned ; with 
a spectroscope of high dispersive 
power, such as one in which the light 
is passed through several prisms, 
this line is seen double. The spec- 
trum of a gas is usually obtained 
by passing electrical discharges 

Fig. 44. — Kirchhoff, the Discoverer of . ' _ . . , , 

the laws of Spectrum analysis. through a Geissler s tube con- 
taining the gas. The narrow por- 
tion of the tube has a very small bore, and the gas in it glows 
brightly. 

3. A gas absorbs from white light passing through it those rays 
which the gas itself when incandescent emits. This law explains ab- 
sorption spectra. When sodium vapor in the flame of the spirit lamp 




Fig. 45. — A Geissler's Tube. 



is interposed between the slit of the spectroscope and the calcium 
light, it absorbs much of the yellow light coming from the lamp 
which would otherwise have fallen upon that place in the spectrum 
where the main sodium line is located. This place in the spectrum 



THE SUN. 



51 



is therefore lighted up only by the light coming from the sodium 
vapor and a portion of the yellow light coming from the lime. 
Hence it looks dark by contrast with the rest of the spectrum which 
is brightly illuminated by the lime light 

It is found that the heated vapor of any elementary substance, 1 
(like sodium, iron, aluminum, oxygen, etc.,) when at a given tem- 
perature and pressure, has for its spectrum a particular group of 
bright lines by which it is distinguished from other elements. 



r 



■ ; 1 
[ 


1 l 1 

mm 


1 1 


-1 


Ill 


: i 

1 1 


1 


: 

I 






J I 




| |j 

M 




i 


k 




1 | 

if ! 

: i - 

: 1 ■' 






1 

Jl 




I 




■Mi 

i : 

■ n ; -' 


! 


1 






1 ! 


M ':■' ! i 
I 1 ! 


. j 
■ 


1 








■ if 


i 
I 


. 


ill III 


1 


i, :' ' 1 


' f ! ! 




ill ill t 



Fig. 46. — A Portion of the Solar Spectrum. 

74.^ The Solar Spectrum. — When sunlight is admitted through the 
slit of a spectroscope, it gives an absorption spectrum, the lines 
being very numerous ; a number of them are shown in Fig. 46. 



mm 




UU.!LuJlll!lSil 

111 I I UL11L J 



Fig. 47. — Correspondence of Bright and Dark Links 
in Two Spectra. 

From the third law we conclude that the light coming from the 
photosphere has passed through some gas or gases on its way to us. 
In order to find out what these gases are, we compare the solar 
spectrum with the bright line spectra of various gases. This is 
done by admitting sunlight through one half of the slit, while the 



1 For a list of the elements, consult any work on chemistry- 



52 



DESCRIPTIVE ASTRONOMY. 



light from some glowing gas is admitted through the other half. 
One who looks through the telescope sees one spectrum above the 
other, as shown in Fig. 47, which exhibits the correspondence 
between bright and dark lines in portions of two spectra. 

We conclude that whenever the bright lines in the spectrum of 
some particular glowing gas correspond to certain dark lines in the 
solar spectrum, that gas is present in the atmosphere of the sun. 

75. Constituents of the Sun : Telluric Lines. — By comparison of the 
spectra of various vapors with that of the sun, it has been shown 
that many of the substances found on the earth exist in the sun 
also. Some of the most commonly known of these are iron, carbon, 
hydrogen, nickel, and copper. No trace has been found of such 
important elements as chlorine, nitrogen, and mercury. But the 
spectra of these, when heated to an enormous temperature, as at the 
sun's surface, may be very different from those produced in our 
laboratories. 

Lockyer l has advanced the theory that substances regarded as 
elements are really compounds which are separated into their con- 
stituents by the intense heat at the sun, so that their spectra are 
much changed. 

Many of the dark lines in the solar spectrum are caused by 
absorption in passing through our atmosphere. These are called 
telluric (tellus, the earth) lines. 



THE SUN'S SURROUNDINGS. 

76. The Chromosphere. — The chromosphere is that portion of the 
sun's " atmosphere " which lies next to the photosphere. It is 
visible during a total solar eclipse, when the moon has hidden the 
photosphere from view; its color is scarlet. It is composed of 
upright filaments, and has the appearance of a stubble-field, the 
" stubble " averaging over five thousand miles in height. Hydrogen, 
helium, and calcium are its principal constituents. 

Helium received its name from the Greek word helios (the sun), 
because it was supposed to exist in the sun only. But when Dr. 
Ramsey 2 was examining a specimen of a species of pitchblende in 

1 J. Norman Lockyer, an English astronomer, who is among the foremost of living 
spectroscopists. 

2 An English physicist, one of the discoverers of argon. 



s 



THE SUN, 




Fiff. 48. 



SOLAR PROMINENCES. 



THE SUN. 53 

1895, h e detected helium in it. It has since been found in certain 
mineral springs in Europe, and in several rare minerals, though 
always in small quantities. It is now known to be widely distributed 
throughout the universe, for lines due to it are in the spectra of 
stars and nebulae. 

77. Prominences or Protuberances. — At the time of a solar eclipse 
many fantastic crimson objects are seen jutting out from the chromo- 
sphere at the sun's limb. They are divided into two classes, the 
cloudlike or quiescent, and the eruptive. 

They are shown in Fig. 48. 

The former are immense irregular masses which overhang the 
chromosphere, looking like the thunder-heads which lazily bask in 
the sunshine on a quiet summer afternoon. Usually they are 
connected with the chromosphere by columns which remind one of 
pictures of terrestrial water-spouts. Sometimes they last a month. 
One has been seen which was 475,000 miles high: its extreme 
apparent breadth was about the same. They are of the same com- 
position as the chromosphere. 

Eruptive prominences are fiery fountains of gas which spurt 
out from the chromosphere. Some fine specimens are shown in 
Fig. 48. One has been known to rise to a height of 350,000 miles. 
In these prominences not only chromospheric matter, but some of 
the vapors of the photosphere are carried up, with velocities which 
baffle comprehension. On May 5, 1892, a velocity of 323 miles 
per second was measured. This eruption probably hurled masses 
of glowing gas entirely away from the sun. 

78. Prominences seen with the Spectroscope. — Prominences are not 
visible with a simple telescope except at the time of an eclipse. But 
if a spectroscope of good dispersive power be attached at the eye- 
end and properly adjusted, one may study the prominences on any 
clear day. (See Fig. 49.) The explanation of this may be found 
in large works on astronomy. 1 

Prominences are distributed all over the sun, but are seen only 
at its limb. Those upon its face are invisible because the photo- 
sphere back of them is so bright. 

79. Prominences and Magnetic Storms. — Prominences, like sun 
spots, are periodic ; their times of maximum and minimum coincid- 

1 See Young's General Astronomy, Art. 324. 



54 



DESCRIPTIVE ASTRONOMY. 



ing with those of the spots. Like the spots, they are associated with 
magnetic storms. 




Fig. 49. — A Spectroscope attached to a Telescope. 



Prof. Young, when observing at a mountain station during the 
forenoon of August 3, 1872, noticed especial activity of prominences, 
jets of unusual brightness being ejected. At dinner time one of the 
party, who had been taking magnetic observations, and who did not 
know what Prof. Young had seen, said that he had been obliged to 



THE SUN. 



55 



desist, because the magnet had swung clear off the scale. Three 
times during the forenoon especially violent disturbances were 
observed, and at those times the magnetic needles in English obser- 
vatories exhibited great fluctuations. 

80. Appearance of the Corona. — At the moment when the last 
ray of sunlight vanishes, in a solar eclipse, there bursts upon the 
vision a pearly radiance of wonderful beauty, which is shown in 
Fig. 50. This is the corona, so called because it is a crown upon 
the Kino; of Dav. 





Fig. 50. — The Corona on July 29, 1878. 

Near the sun it is almost dazzling in brightness, but it fades away 
into faint streamers and tufts of light which sometimes extend to 
great distances. Observers on the summit of Pike's Peak, in 1878, 
saw streamers 9,000000 miles in length. In the telescope the inner 
bright part of the corona is seen to be composed of innumerable 
fine filaments, like the dishevelled blonde tresses of some mountain 
nymph. Fig. 5 1 is from a photograph of the corona. The corona 
differs widely in appearance at different eclipses. 

During 1895, Mr- D. E. Packer 1 made the capital discovery that 



1 Of South Birmingham, England. 



56 



DESCRIPTIVE ASTRONOMY. 



the corona could be photographed on any clear day through a thin 
metallic screen. Coronal light, like the Rontgen rays, penetrates 
such screens. The photographs show that there is an intimate con- 
nection between the corona and active sun spots, as every promi- 




Fig. 51. — The Corona, photographed on Dec. 21, 1SS9. 

nent filament points toward some spot. Many of the filaments are 
twisted, like a corkscrew. 1 

81. Schaeberle's Theory of the Corona. — Prof. Schaeberle 2 has ad- 
vanced a theory that the corona is caused by the ejection of numer- 
ous streams of matter, driven by forces which are most active near 
the centre of the zones in which spots are found. Owing to the 
sun's rotation these streams are curvilinear, and appear to interlace. 

1 Mr. Packer's work still (July, 1896) awaits confirmation by other astronomers, and 
may prove to be illusory. 

2 J. M. Schaeberle, Astronomer at the Lick Observatory, Mt. Hamilton, Cal. 



THE SUN. 



57 



The variations in the form of the corona at various eclipses are 
partially explained by the fact that our point of view is continually 
changing, as the earth pursues its annual journey about the sun. 





Fig. 52. — Illustrations of Schaebf.rle's Theory of the Corona. 

Prof. Schaeberle took a ball to represent the sun, and thrust into 
it a large number of needles, in the regions corresponding to those 
where spots on the sun are most numerous. The ball was then 
placed in several positions, and photographed : two of the results 
are shown in Fig. 52. 




Fig. 53«. — A Drawing of the Corona. 

The photographs of the eclipse of April 16, 1893, are thought 
by Prof. Schaeberle to confirm his theory. 

82. Nature of The Corona. — The corona gives two spectra, one 
made up of bright lines, the other continuous. The bright line 
spectrum comes from a glowing gas. The continuous spectrum may 
come from incandescent solid or liquid particles scattered through 



5« 



DESCRIPTIVE ASTRONOMY 



the corona, or from the light of the photosphere reflected from the 
materials of the corona. 




Fig. 53^. — A Drawing of the Corona. 

The bright line spectrum, when carefully examined, reveals the 
presence of an unknown element, which has been called coronium ; 
hydrogen is also found. Xo complete explanation of the phenomena 





Fh 



A Drawing of the Corona. 



Fig. 53</. — A Photo- 
graph of the In- 
ner Corona. 



exhibited by the corona has yet been found, but it is not improbable 
that electrical action may account for many of them. Fig. 54 shows 
some effects produced by Dr. Pupin, 1 by electrical discharges around 



1 M. I. Pupin, Columbia College, New York. 



THE SUN. 



59 



a brass sphere placed in a globe from which the air was largely ex- 
hausted. They are strikingly like coronal forms. Similar experi- 
ments have been made in Germany by Dr. Ebert and Prof. Wiede- 
mann. These experiments show the raylike structure and silvery 
light of the corona, the dark rifts which are frequently seen extend- 
ing from the sun's limb to the limit of the corona, and the abnormally 
long streamers which have graced the sun during certain eclipses. 




Fig. 54. — Electrical Appearances similar to the Corona. 

83. Light of the Sun. — Under the clear sky of Colorado, a news- 
paper may be read by any person of normal eyesight by the light 
of the full moon. How 100,000 full moons, crowding the vault of 
the heavens would blind us by their radiance ! Yet the sun gives 
us six times as much light. If an electric arc light be placed be- 
tween the eye and the sun, and both be viewed through a dark 
glass, the arc light will appear as a dark spot on the face of the sun. 
Since the earth receives only 2 ,200,000000 °f the light radiated by 



60 DESCRIPTIVE ASTRONOMY. 

the sun, the amount of light radiated in all directions by the sun is 
2,200,000000 times as great as that which the earth receives. (See 
exercise 3, at the end of this chapter.) This inconceivable quantity 
of light is shot through space with a velocity of 186,330 miles per 
second. Were the sun divested of its atmosphere, it would probably 
be three times as bright, and blue in color. The blue rays are now 
strongly absorbed by its atmosphere. 

84. Heat of the Sun. — By letting the sun shine for a given 
length of time upon the blackened cover of a box filled with a 
known quantity of water, and by noting the rise of temperature in 
the water, it is possible to find approximately the amount of heat 
received by the earth from the sun. Calculation based on such 
measurements has shown that the sun sends to the earth every 
second enough heat to raise 600,000000 tons of ice water to the 
boiling point. 

Imagine that a gigantic fire-engine was throwing at the sun a 
stream of water 75,000 miles in cross section, at the rate of 1,000 
miles a second. The water would be turned into steam as fast as it 
advanced, if the entire heat of the sun were concentrated upon it. 
Stationary engines have been run by concentrating the sun's heat 
by means of huge reflectors. In case some economical way of 
storing and distributing heat energy were discovered, solar engines 
might take the place of coal-burning ones. 

Langley : says that, " even on such a little area as the island of 
Manhattan, or that occupied by the city of London, the noontide 
heat is enough, could it all be utilized, to drive all the steam engines 
in the world." 

85. Causes of the Sun's Radiation : Combustion : Meteoric Theory. — 
It is certain that the outpour of heat and light is not kept up by 
mere combustion. Had the sun been a solid mass of the best 
anthracite, burning swiftly enough to produce the known supply of 
heat, less than 6,000 years would have been required for its complete 
consumption. 

There has been a theory that a continual rain of small bodies 
falling upon the sun from adjacent space keeps up the supply of 
heat. We see evidences of such production of heat, when a cannon 

1 Dr. S. P. Langley, Secretary of the Smithsonian Institution, Washington, D. C. 



THE SUN. 



61 



ball strikes an armor plate, and both are heated by the impact. 
This is known as the " meteoric theory." While the sun doubtless 
receives some of its heat from such a pelting, the most careful inves- 
tigations show that only a minute fraction of its heat can come from 
such a source. For if the sun be thus bombarded, why not the 
earth, though to a much less degree, on account of its smaller size 
and feebler attraction? 

Calculation has shown that, upon this theory, each square 
mile of the earth would be bombarded by fifty tons of missiles 
every day. 

86. The Contraction Theory. — If a body be dropped from the top 
of a high tower, heat will be produced when its motion is arrested 




Fig. 55. — Production of Heat by the Action of Gravity. 



by striking the earth. If it be made to fall slowly, by being used as 
a weight to drive a machine, as in Fig. 55, heat will still be pro- 
duced. In the machine shown in the cut, the revolution of the 
paddles heats the water. Now, we conceive that the entire mass of 
the sun is shrinking slowly, each particle (except the one at the 
centre) gradually falling inward. This process will generate heat. 
So enormous is the sun's mass that the rate of contraction necessary 
to keep up the supply of heat is very slow, being only ten inches a 
day. The amount of contraction during the past 6,000 years would 
not be noticeable, even with the best modern telescope. This theory 
is generally accepted by astronomers as the best which has been 
advanced. 



62 DESCRIPTIVE ASTRONOMY. 

87. Past and Future of the Sun. — If the sun has been radiating 
heat uniformly in all directions, at the same rate as now, during its 
entire past, and if the heat has been kept up by contraction alone, 
however large it may originally have been, in less than 18,000000 
years it would have shrunk to its present dimensions. Since the time 
when its diameter was equal to that of the orbit of Mercury, it has 
radiated over eighty times as much heat as previously, according to 
this theory. By the use of similar assumptions it has been guessed 
that the sun will not give enough light and heat to supply the needs 
of man for more than 10,000000 years hence. These figures might 
be awe-inspiring, if the foundations on which they rest were more 
substantial than " the baseless fabric of a vision." For it is extremely 
improbable that we can reason with any approach to exactness from 
the slender data at our disposal. The sun may be much older or 
younger. As Sir William Thomson 1 once said, in a lecture on this 
subject, " After all, we don 't know anything about it." 

88. The Sun's Constitution. — The interior of the sun is supposed 
to be gaseous on account of the intense heat, the gases being 
extremely compressed by the weight of the huge solar bulk. 

Surrounding this interior is the photosphere, a cloud shell formed 
of vapors which, though they have been condensed by exposure to 
the cold of surrounding space, are yet very hot. 

The chromosphere comes next, composed of gases not so easily 
condensed as the materials of the photosphere ; chief among these 
is hydrogen. 

Mingled with the chromosphere, but extending to vastly higher 
elevations, is the mysterious corona, made up of rare gases, through 
which are scattered finely divided particles of matter, which might 
remind us (if we could see them) of motes floating in a sunbeam. 

EXERCISES. 

89. 1. If light travelling 186,330 miles per second consumes 8 m. 
19 sec. in coming from the sun to us, find the sun's distance from 
the earth. 

2. In § 83 it is estimated that 100,000 full moons would more 
than fill the visible hemisphere of the sky. Let us find out how the 

1 Now Lord Kelvin, the great mathematical physicist. 



THE SUN. 63 

number was computed. The moon is 240,000 miles from us, and 
its radius is 1,080 miles. Imagine that the visible sky is a hemi- 
spherical surface, the radius of which is 240,000 miles, and that the 
moon is a circle, the radius of which is 1,080 miles, located on the 
hemispherical surface. The area of a hemisphere = 2X 3.1416 X 
the square of its radius. The area of a circle = 3.i4i6xthe square 
of its radius. How many times the area of the circle is the area of 
the hemisphere? 

3. In § 83 it is stated that the earth receives only 2,200,000000 
of the light and heat sent out by the sun. Imagine a huge soap- 
bubble, the centre of which is at the sun, its radius being the distance 
from the sun to the earth, 93,000000 miles. Change the film to a 
thin crystal shell in which an emerald 8,000 miles in diameter is set, 
to represent the earth. Remove the emerald, leaving a circular 
hole 8,000 miles across. The light which strikes the crystal sphere 
in one second equals the total light emitted by the sun in one 
second. The light which streams through the hole in one second 
equals the amount received by the earth in one second. Hence, 
as the area of the hole is to the area of the sphere, so is the amount 
of light the earth receives in one second to the total light given out 
by the sun in one second. 

The area of the surface of a sphere = 4X 3.1416 X the square of 
its radius. The area of a circle = 3.1416 X the square of its radius. 
Compute the fractional part of the sun's radiation which strikes the 
earth. 

4. Why cannot the prominences and corona be seen with a good 
telescope on any bright day? 

5. What change does the vapor which is shot off from the sun 
(§77) undergo, in passing through space? 



64 DESCRIPTIVE ASTRONOMY. 



CHAPTER V. 

THE EARTH. 

' ; The earth, 
Though in comparison of heaven so small, 
Nor glistering, may of solid good contain 
More plenty than the sun that barren shines, 
Whose virtue in itself works no effect, 
But in the fruitful earth ; there first received 
His beams, inactive else, their vigor find." 

Milton. 

90. Dimensions and Shape. — The earth is a globular body, nearly 
8,000 miles in diameter. The surface of the ocean is not truly spher- 
ical, but bulges at the equator. If a soft rubber ball be spun rapidly 
on an uncarpeted floor, it will assume a form like that of the earth : its 
shortest diameter will be that on which, as an axis, it rotates. This 
is due to the tendency of every particle of matter which is whirling 
around a centre to fly away from it. Mathematicians call the earth 
an oblate spheroid. Such a solid is formed by revolving an ellipse 
(§ 96) about its shortest diameter as an axis. 

91. Direction of the Plumb-line. — A string, at the lower end of 
which a plumb-bob hangs, is a plumb-line. If the earth were truly 
spherical and homogeneous, and did not rotate, its attraction for 
the bob would cause the line to point directly to its centre. In 
Fig. 56, PP' is the earth's axis, and EQ its equator. When the earth 
whirls, the plumb-bob at O tends to fly directly away from C, in the 
direction of the line CQ prolonged. But gravity pulls the bob 
directly toward C, and overcomes its tendency to fly away. At D, 
on account of the earth's rotation, the bob tends to fly away from 
the axis PP', in the direction indicated by the arrow A. This ten- 
dency causes the plumb-line, instead of pointing toward C, to swing 
a trifle, taking the position shown. At P, since the plumb-line is 
in the prolongation of the earth's axis, the rotation causes no side- 
wise swing. Hence, at the poles and at any point on the equator the 



THE EARTH, 



65 



plumb-line points towards the earth's centre (if the earth be homoge- 
neous); at other places it does not point quite towards the centre. The 
plumb-line is perpendicular to the surface of still water. 




Fig. 56. — Direction of the Plumb-line, 

92. How the Earth's Diameter is Found. — While the details of this 
process are too difficult for us to understand at this stage of our 
progress, we may get a notion of the principles involved by assum- 
ing that the earth is a perfect sphere, and that a plumb-line points 
toward its centre. In Fig. 57, C is the earth's centre, and AB an 
arc of a meridian. At A, AP is drawn toward the north celestial 
pole, and AZ in the direction of the plumb-line. At B, BZ' is 
drawn " plumb," and BD is made parallel to AP. BE is parallel to 
AZ. Now the angle PAZ = DBE, because their sides are parallel. 
For the same reason, — 



EBZ' = ACB. 
Then ACB = EBZ 7 , 

= DBZ' - DBE, 
= DBZ' - PAZ. 

5 



66 



DESCRIPTIVE ASTRONOMY. 



An astronomer at B measures the angle DBZ' ; then, using his 
utmost skill and refined apparatus, he determines the length of AB 
in feet or meters. On arriving at A, he measures the angle PAZ. 
Subtracting one of these angles from the other, he gets the value of 
ACB, as explained above. If ACB were one degree, the circum- 
ference of the sphere would be 360 times the length of AB in feet 
or meters. Geometry teaches that the circumference divided by 
3. 141 6 gives the diameter. 

The real spheroidal shape and the lengths of the polar and equa- 
torial diameters are deduced from measurements made in various 
parts of the world. 1 

N 

0- %> ,i 

\ Vi 





Fig- 57- 



Fig. 



Latitude and Longitude. 



93. Latitude and Longitude, if the Earth were a Perfect Sphere. — 

Assuming for a moment that the earth is a perfect sphere, we get 
the following definitions. 

The terrestrial meridian of any point on the earth's surface is the 
circle drawn through the point and the poles. In Fig. 58, NGG'SR 
is the terrestrial meridian of Greenwich, if G represent that place. 

The latitude of any point on the earth is the arc of its meridian, 
between the equator and the point. The arc is measured in de- 
grees. A city in 30 north latitude is one third of the way from 

1 The lengths of the diameters are respectively : — 

Polar 7,901.476 miles. 

Equatorial ... 7,926.592 miles. 



THE EARTH. 



6 7 



the equator to the north pole. A place in 45 ° south latitude is half 
way between the equator and the south pole. 

The longitude (from Greenwich) of any point on the earth is 
the arc of the equator embraced between the meridian of the point 
and the meridian of Greenwich. It is reckoned either east or west 
of the Greenwich meridian. Sometimes it is expressed in degrees, 
but more often in hours, by putting 360 equal to 24 h. Thus 
1 5° equals 1 h. ; a place in longitude 105 west of Greenwich is said 
to be 7 h. west of Greenwich. 

94. Latitude and Longitude accurately Defined. — Astronomers 
are accustomed to regard the earth as a perfect spheroid (§ 90), 
making allowances, when ne- 
cessary, for the irregularities < /\ p 
caused by the unevenness of 
its surface. 

The astronomical latitude of 
any point on the earth's surface 
is the angle made by the plumb- 
line suspended at that point F J- — ^J — — 1Q 

with the plane of the equator. 
In Fig. 59, the astronomical 
latitude of D is the angle DAE. 

The plane of the meridian of 
any point on the earth's surface 
is the plane passing through 
the point and the poles. Any 
two meridian planes intersect, 

their line of intersection being the earth's axis. In Fig. 59, PP' is 
the line of intersection of the planes PGP' and PEP'. The angle 
between these planes is (§ 19) equal to ECB, because EC and BC 
are both perpendicular to PP'. The angle ECB is measured bv 
the arc EB. 

The longitude (from Greenwich) of any point on the earth's sur- 
face is the angle made by its meridian with the Greenwich meridian ; 
or it is the arc of the equator lying between the two meridians. 
Thus, in Fig. 59, the longitude of D, reckoned from G, is the angle 



1 / 


) / 
/g 

AS 


_ \r \ 

c 1 Nj 






_X^j 


\ B 




iy 




>' 



Fig. 59. — The Astronomical Latitude. 



between the planes PGP' 
measured by the arc EB. 



and PEP', which, as 



have found, is 



68 DESCRIPTIVE ASTRONOMY. 

Latitude is measured in degrees ; longitude in degrees or hours, 
as stated in the last section. 

95. Variation of Latitude. — During the past few years it has 
been shown that the latitudes of various observatories are not con- 
stant, but change by slight amounts, according to a law which has 
been pretty thoroughly determined by Dr. S. C. Chandler. 1 The 
change is due to a "wobbling" of the earth upon its axis, so that 
the north and south poles do not remain at the same points on the 
earth's surface. This movement of the poles amounts to a few 
yards in the course of a year, and takes place in a very sinuous 
path. It is believed to be due to slight changes in the earth's 
form, due to the varying forces which act upon it. A rude illus- 
tration is given by a base ball, which is usually rotating about some 
axis just before it is struck by a bat; immediately after the stroke, 
it commonly rotates about an entirely different axis. 

THE ECLIPTIC AND THE ZODIAC. 

96. The Orbit. — The earth completes its circuit about the sun in 
a year, moving in an ellipse. This curve may be drawn as shown in 

Fig. 6o. A string runs from F around 
the pencil at P to ¥'. The pencil, being 
moved in such a way that the string slips 
around it and is continually kept taut, 
describes the curve. F and F' are the 
foci. A A' is the major axis ; BB' is the 
minor axis ; C is the centre The sun is 
not at the centre of the earth's orbit, but 
at one of the foci. When the earth is 

Fig. 6o. — An Ellipse. 

nearest the sun, it is said to be in peri- 
helion ; when farthest away, in aphelion. Half the major axis is 
called the mean distance (CA) ; the mean distance of the earth is 
93,000000 miles. The earth travels most swiftly when in perihelion, 
and with the least velocity at aphelion. 

97. The Ecliptic. — If the plane of the earth's orbit be extended 
till it cuts the celestial sphere, their intersection will be a circle, 
known as the ecliptic. In Fig. 6i is a pond of water in which a 

1 Of Cambridge, Mass. 




THE EARTH. 69 

croquet ball is floating, half submerged. The surface of the water 
represents the plane of the ecliptic, and the croquet ball the earth, 
the centre of which is just in the plane. The point S, near the 
centre of the pond, represents the sun's centre. 

If the earth's axis were perpendicular to the plane of its orbit, 
the planes of the equator and ecliptic would coincide. But the 
axis is tipped so as to make an angle of 23 27' with the perpen- 




Fig. 61. — Illustration of the Ecliptic. 

dicular. (See Fig. 62.) The plane of the equator, being per- 
pendicular to the axis, is also tipped, and makes the same angle 
with the plane of the ecliptic. This angle has been named the 
obliquity of the ecliptic. 

98. The Equinoxes. The Sun's Yearly Path. — The planes of the 
ecliptic and equator, when extended, cut the celestial sphere in two 
circles, which cross each other at two opposite points (A and B) in 
Fig. 63. The points are the vernal and autumnal equi?zoxcs respect- 
ively. It is evident, from Fig. 61, that a straight line drawn from 
the earth's centre through that of the sun, and prolonged to meet 
the celestial sphere, will strike at some point of the ecliptic. As the 
earth moves in its orbit around the sun, the other end of this line 
moves along the ecliptic. 

To an observer on the earth, the sun appears to be on the sur- 
face of the celestial sphere, at the end of the line just mentioned. 
Hence the sun seems to us to creep along the ecliptic, taking a year 
to make one complete circuit. On or near the 20th of March, the 



7o 



DESCRIPTIVE ASTRONOMY. 



sun is at A; this point is called an equinox, because at that time 
the days and nights are equal, as will be demonstrated later. Six 
months afterwards, about September 22d, the sun is at B, the 
autumnal equinox. 




ECLIPTIC 




Fig. 62. — The Obliquity of 
the Ecliptic. 



Fig. 63. — The Ecliptic and 
the Equator. 



99. The Solstices : The Sun's Eastward Motion. — The point C in 
Fig. 63, midway between A and B, is the summer solstice. The sun 
is at this point about June 20th. In travelling from A, the vernal 
equinox, to C, the summer solstice, the sun keeps getting farther 
away from the celestial equator every day, and nearer to the north 
pole. While travelling from C to B, the sun continually lessens its 
distance from the celestial equator. After passing through the 
autumnal equinox, it is between the south celestial pole and the 
equator, and gets farther south of the equator every day until it 
reaches D, the winter solstice, about December 21st. 

When going from D to A, it is approaching the equator every 
day. It must be diligently remembered that during the entire year, 
the sun, whether north or south of the celestial equator, is contin- 
ually moving eastward along the sphere. If one could look right 
past the sun and see the stars beyond, the amount of the sun's 
motion among the stars in a day would be very plain to the naked 
eye. Fig. 64 shows such a view of the sun's daily motion. 

100. The Zodiac. — The zodiac is a belt 16 broad, encircling the 
sky, like the colored band on a croquet ball. The ecliptic is its 
central line. Twelve constellations lie in this belt or zone. They 
are : Aries, Taurus, Gemini ; Cancer, Leo, Virgo ; Libra, Scorpio, 
Sagittarius ; and Capricornus, Aquarius, Pisces. The moon and the 



THE EARTH. 



71 



planets are always to be found in the zodiac. The signs of the 
zodiac are twelve arbitrary divisions of it, each of which is 30° long. 
The first three of them stretch from the vernal equinox to the 
summer solstice. The signs have the same names as the constella- 



ECUPTIC 







Fig. 64. — The Sun's Daily Motion among the Stars. 

tions just mentioned, but do not coincide with them. The expres- 
sion, " The sun enters Aries," found in almanacs, means that the 
sun passes the vernal equinox. 

101. Is the North Celestial Pole Fixed? — When a heavy top spins 
rapidly on a smooth surface, its axis keeps the same direction, no 
matter how much the top wanders around. The successive posi- 
tions taken by the axis of the 

top are parallel lines. As the 
earth goes spinning around the 
sun once a year, all the posi- 
tions of its axis from moment 
to moment are parallel lines. 
We here neglect certain minute 
tippings caused by the attraction of other bodies for the earth. 
These parallel lines when extended appear to strike the celestial 
sphere at the same point, because of the infinite distance of the 
sphere from us. This explains why the north celestial pole can be 
found at the same place among the stars (§ 18) during the entire 
year. It does not move enough during his lifetime to attract the 
attention of one observing with the naked eye. 

102. Are the Celestial Equator and Ecliptic Fixed? — Since the plane 
of the earth's equator is perpendicular to its axis, all the positions 




Fig. 



A Spinning Top. 



72 



DESCRIPTIVE ASTRONOMY. 



So 



<& 



<& 



9S 



which it takes during a year will constitute a series of parallel planes. 

These planes, being extended to the celestial sphere, cut it in great 

circles, which appear to us to 
coincide on account of their 
infinite distance from us. Since 
the earth's axis is tipped a little 
this way and that, as mentioned 
in the last article, the equator 
suffers slight shiftings. But for 
purposes of naked eye obser- 
vation, we regard the celestial 
Its place among them is shown 



5 

9S 



<& 



Fig. 



66. — Successive Positions of the 
Earth's Equator. 



equator as fixed among the stars, 
on the maps. 

Similarly the ecliptic is to be regarded as a circle practically 
fixed among the stars. 



DAY AND NIGHT: THE SEASONS. 

103. The Length of The Day. — Everybody knows that in summer 
the days are long and the nights short, and that the case is reversed 
in winter. We refer now to middle latitudes, leaving the polar 
regions and the equator out of consideration for the moment. 

One may get a clear grasp of this matter by a simple experi- 
ment. Take an orange, with a knitting needle thrust through it to 
serve as an axis of rotation, and mark upon it with a penknife the 
equator, together with three or four circles on each side of the 
equator and parallel to it. Submerge half the orange in a basin of 
water in the position shown in 
Fig. 6y. The surface of the 
water represents our horizon, 
and the upper part of the or- 
ange the half of the celestial 
sphere visible to us at any mo- 
ment, PP ; being its axis and EQ 
its equator. It is plain that just 
one half of the equator is below 
the horizon. Hence, if any ce- 
lestial object is on the equator, as the sphere turns, the object will 
be above the horizon for twelve hours and below for the same length 




Fig. 67. — An Orange Half Submerged. 



THE EARTH. 73 

of time. More than half of every circle between P and EQ is above 
the horizon. Therefore, if any celestial object be north of the 
equator, it will be above the horizon more than twelve hours out 
of the twenty-four. If it be near the north celestial pole, it will 
be above the horizon all the time. Less than half of every circle 
between P' and EQ is above the horizon. Consequently any celestial 
object south of the equator is above the horizon less than twelve 
hours out of the twenty-four. If it be near the south celestial pole, 
it will never rise above the horizon. 

On March 20th the sun is near the vernal equinox all day ; and 
since this point is on the equator, it rises nearly at the east point of 
the horizon and sets nearly at the west point, so that both day and 
night are almost exactly twelve hours long. After March 20th the 
sun, creeping along the ecliptic, gets farther and farther north every 
day until June 20th, so that the days continue to grow longer and the 
nights shorter until that date, which is the longest day of the year. 

After June 20th the sun approaches the equator and the days 
shorten and the nights lengthen until September 22d, when the sun 
reaches the autumnal equinox, and the days and nights are again 
equal. The sun then passes south of the equator, and the days 
grow shorter till December 21st, when the sun is farthest south. 
The sun approaches the equator during the next three months, so 
that the days continually lengthen until the days and nights again 
become equal. 

104. Day and Night at the Equator. — In Chapter II. we learned 
that, if a man lived at the equator, the celestial poles would be on 
his horizon at the north and south points. The orange used in the 
last section is now to be placed in the water, with the knitting needle 
lying horizontal in the liquid surface. Then every circle marked 
on the orange, as previously described, will be half submerged, and 
the sun, wherever it may be on the sphere on any given day, will be 
above the horizon twelve hours, and below for the same number of 
hours. 

105. Day and Night at the Poles. — The horizon of a man at the 
north pole would be parallel to the terrestrial equator. Both planes, 
when extended to the celestial sphere, would seem to cut it in the 
same circle, the celestial equator. The north celestial pole would 
be in the zenith. Therefore, if the sun were north of the equator 



74 



DESCRIPTIVE ASTRONOMY. 



it would be above the horizon, and if south of the equator it would 
be below the horizon. For the possible polar bears or seals in 
that locality there is continuous day from March to September, and 
night for the remaining months of the year. During much of the 
night there is a strong twilight, because the sun is not far below 
the horizon. 

106. The Midnight Sun. — In Fig. 68, PP' is the earth's axis, and 
EQ is the equator. O is the situation of an observer who is in a 
high northern latitude. OA, drawn from the observer's position at 
O, parallel to the earth's axis, is directed toward the north celestial 
pole. OZ points toward the zenith, and NS represents the horizon. 
Suppose that the observer is in yo° north latitude, then the angle 
OCQ will be 70 , and OCP will be 20 . But OCP= AOZ. Since 
OA points to the pole and OZ to the zenith, and the angle AOZ is 
small, the pole is near the observer's zenith. 





N 

MIDNIGHT 



Fig. 69. — The Midnight Sun. 

The appearance of the celestial sphere, as seen by an observer at 
O is shown in Fig. 69. NESW is the horizon ; EQW is half of the 
celestial equator ; P is the north pole, and OP is the apparent rota- 
tion axis of the sphere. In June the sun is so far north of the 
equator that its daily path through the sky, ABCD, lies entirely 
above the horizon, so that it is visible even at midnight ; this may 
be made very plain by using the orange of § 103. 

107. The Seasons in Middle Latitudes. — We consider first a place 
at a north latitude of about 45 . The change of seasons is due 
principally to two causes. 1. The sun is above the horizon during 



THE EARTH. 



75 



more hours on a day in summer than on a day in winter. 2. The 

sun heats the earth's surface at any place the more powerfully the 

higher up it is in the 

sky. This is shown in 

Fig. 70, where a bundle 

of rays from the sun, 

coming nearly vertically 

downward, heats up a 

square foot, while an 

equal bundle, striking 

obliquely, spreads its 

heat out over a greater 

surface ; consequently 

it heats each square inch 

of the surface less. The 

oblique rays also traverse a longer path in the atmosphere, and are 

more absorbed by it, before reaching the ground. 

108. The Seasons at the Equator. — The change of seasons is much 
less marked than in middle latitudes : for the sun is above the 
horizon the same time (twelve hours) every day of the year, and its 
rays come down nearly vertically at noon, throughout the year. 




Fig. 70. — Effect of the Slant 
of the Sun's Rays. 



THE PRECESSION OF THE EQUINOXES, AND THE CALENDAR, 

109. Attraction of the Earth's Equatorial Ring. — Since the earth 
bulges at the equator, we may consider it as a true sphere, around 

the equatorial regions of which an 
extra ring of matter has been placed. 
This conception is rudely represented 
in Fig. 71. The attraction of the 
earth and moon upon this ring-like 
protuberance cause the precession, 
which we proceed to explain. Sup- 
pose the moon to lie in the direction 
of the arrows, and by its attractive 
force to be tugging away at the ring. 
Its pull on the half of the ring at the 
left (in the figure) of C tends to tip the earth, so that P will move 




71. — The Earth's Equa- 
torial Ring. 



7 



DESCRIPTIVE ASTRONOMY. 



to the left. Its pull on the half of the ring at the right of C tends 
to tip the earth in the other direction, so that P will move to the 
right. But since the left half of the ring is nearer the moon than 
the right half, it is pulled the more strongly, and therefore P will 
move slowly toward the left. In consequence of this attraction, 
the earth will tend to turn slowly about an axis going through C, 
but perpendicular to the plane of the page on which Figure 71 is 
drawn. But meanwhile the earth is spinning with prodigious 
energy about the axis P P\ and the moon's attraction on the ring 
produces only a slight disturbance of this energetic rotation. 

110. Illustration with a Top. — If a top, which is not spinning, be 
placed in a slanting position on a floor, the force of gravity pulling 
in the direction of the arrow in Fig. J2, will cause it to fall over; in 
falling it will move just as if it were turning about a line AB, drawn 




Fig. 72. — A Leaning Toi 




Fig. -J2- ~ Positions of the Axis of the Top. 






on the floor, as an axis. When the top is spinning swiftly in an 
inclined position, it rotates about the axis PS, and gravity is at the 
same time trying to make it rotate about AB. Any one who has 
watched a top knows that, under these circumstances, the axis PS 
moves slowly around, taking the successive positions PS, PS', PS", 
etc., Fig. 73. PZ is perpendicular to the floor. The more swiftly 
the top spins, the slower is this motion of PS around PZ. 

111. The Earth compared with the Top. — Both spin rapidly. The 
force of gravity attempts to tip the axis PS of the top. The pulls 
of the sun and moon on the equatorial protuberant ring of the earth 
tend to tip its axis, PP'. The axis of the top swings around PZ, a 



THE EARTH. 



77 



perpendicular to the plane of the floor. The earths axis swings 
around a perpendicular to the plane of the ecliptic as shown in 
Fig 74 CA is perpendicular to the ecliptic; PC, the earth s ax.s, 
takes successively the positions PC, P'C P"C, etc., 25,800 years 
being required for a complete journey around the circle PP P V . 




Fig. 74- 



-The Precessional Motion of the Earth's Equator. 



The point where the equator cuts the plane of the ecliptic likewise 
moves around, taking the positions V and V successively On 
extending the planes of the equator and the ecliptic to the sky, the 
point V, where the celestial equator and ecliptic intersect, becomes 
the vernal equinox. 



7» 



DESCRIPTIVE ASTRONOMY. 



DENEB 



POLARI5 



Hence, the venial equinox moves slowly along the ecliptic, west- 
ward, taking 25,800 years for a complete revolution. The autumnal 
equinox does likewise. This is the precession of the equinoxes. 

This motion of the equinoxes may be made plain by using the 
orange described in § 103, and representing the ecliptic by a sheet 
of paper in which a circle a little larger than the orange has been 
cut. Then, by moving the orange in a way suggested by Fig. 74, 
the motion of the equinoxes becomes easily visible. 

112. Some Effects of Precession. — On account of precession the 
north celestial pole, moving as described in the preceding section, 
comes near different stars as the centuries pass away. About 

twelve thousand years 
hence, the north pole 
will be so near the 
brilliant star Vega, 
in the constellation 
of the Lyre, that it 
will be called the pole 
star. 

The path of the 
pole among the stars 
is shown in Fig. 75. 
Polaris will be only 
half a degree from 
the pole in the year 
2000 

The non-coinci- 
dence of the sign Aries with the constellation Aries is due to 
precession. For the sign Aries begins at the vernal equinox, 
which, as we have seen, shifts slowly westward among the stars. 
It is now in the constellation Pisces. If Greenwich and its merid- 
ian moved perceptibly on the earth's surface, the longitudes of all 
other cities would be changed. If the earth's equator kept shift- 
ing its position on the surface, the latitudes of cities would keep 
changing. In the same way the shifting of the celestial equator 
and vernal equinox, by precession, causes changes in the right 
ascensions and declinations (which correspond to longitudes and 
latitudes of cities) of the stars. 




Fig. 75. — The Path of the North Celestial 
Pole among the Stars. 



THE EARTH. 



79 



Since the vernal equinox moves a little westward every year, 
the sun in his apparent annual march eastward reaches the vernal 
equinox sooner than if it were fixed. This makes the year twenty 
minutes shorter than it would be otherwise. 

113. Different Kinds of Years. — The sidereal year is the time 
required for the earth to make a complete revolution about the sun. 
If the sun, as seen by us, is now in line with some fixed star, a 
sidereal year must elapse before it will get into line with the same 
star again. The length of the year is 365 d. 6 h. 9 m. 9 sec. 



5TAR 




Fig. 76. — Different Kinds of Years. 



The tropical year is the time which elapses between two suc- 
cessive passages of the sun through the vernal equinox. Suppose 
that the vernal equinox, when the sun appears to be in it, in March, 
1900, is at V in Fig. 76. As the earth moves from E' to E", E"\ 
etc., the sun appears to move from V through A to B, and so on. 
But meanwhile the vernal equinox, because of precession, has been 
moving westward a slight distance, so that the sun will meet it in 
March, 1901, at V. 



80 DESCRIPTIVE ASTRONOMY. 

A sidereal year will be completed when the sun reaches V again. 
A tropical year is therefore shorter than a sidereal year. Its length 
is 365 d. 5 h. 48 m. 46 sec. 

114. The Julian Calendar. — Julius Caesar found the Roman calen- 
dar in great confusion. It was decidedly complex, and the priests, 
whose duty it was to regulate the religious festivals in accordance 
with it, sometimes introduced alterations capriciously, to subserve 
their own interests. Matters had come to a pretty pass, in Roman 
eyes, when a festival of Bacchus must be celebrated while grapes 
were green. Acting upon the advice of Sosigenes, a noted Alexan- 
drian astronomer, Caesar ordained that three years out of every four 
consist of 365 days, the fourth being 366 days long. He did this 
because the year was known to be about 365^ days in length. If the 
number of a year be divisible by 4 it is a leap year, and contains 366 
days according to the Julian calendar. He also directed that the 
year begin on Jan. 1, instead of in March, as before. 

This Julian calendar went into effect in 45 B. c, and is still 
employed in Russia. Dates reckoned according to it are now 
called " Old Style." 

115. The Gregorian Calendar. — The true tropical year being 363 d. 
5 h. 48 m. 46 sec. in length, the Julian year of 365 d. 6 h. is too long 
by 1 1 m. 14 sec. This discrepancy amounts to over 3 days in 400 
years. In 1582 Pope Gregory XIII. introduced a reform which 
dropped 3 days in every four centuries. This was accomplished by 
ordering that the year which rounds out each century ( 1 300, 1 8oo> etc.) 
should be a leap year only when its date number is divisible by 400. 
Thus 1600 was a leap year according to this calendar, while 1700 
was not. Accordingly three out of every four century years are 
not reckoned as leap years, and 3 days are thus dropped from the 
Gregorian calendar which are retained in the Julian. At the time of 
the famous Council of Nice (A. D. 325) the sun was in the vernal 
equinox on March 21st; but in 1582 the same event happened on 
March nth, because of the imperfection of the Julian calendar. 
The Pope therefore ordered that the 10 days lost should be made 
up by calling the day following October 4th October 15th. 

The new calendar was at once adopted by all nations which 
recognized the Pope's authority. In England, the Old Style was 
used until 1752, when by act of Parliament the New Style was intro- 



THE EARTH. 



8l 



duced, in the face of opposition and rioting on the part of some 
of the people, who acted as though they believed that by the change 
of date from September 3d to September 14th, eleven days were to 
be stolen from them. It is now quite common in Russia to write a 
date according to both styles : thus, Mar. 3 /i 5 . 



ABERRATION AND REFRACTION. 

116. Aberration of Light. — One who walks briskly along the 
street, when the rain is descending vertically, does not hold his 
umbrella straight up, but slants it forward. Were he to stand, 
holding a tube vertical, raindrops would pass through it without 
touching its sides. But if he walked briskly, still holding the tube 
upright, a drop of rain entering at the top would strike against the 
side. However, if he slanted the tube forward at the proper angle, 
drops would go through freely. 
While the drop was failing the dis- 
tance AB in Fig. 77, the man would 
walk a distance equal to CB. 

In the same way, if the earth 
were still, and a man pointed a 
telescope directly at a fixed star, 
the rays from the star would come 
down through the telescope tube 
and emerge at the eyepiece. But 
since the earth moves, it is neces- 
sary to slant the telescope by a 
minute angle, so that rays from 
a star, after passing through the 
object-glass, may come out at the 
centre of the eyepiece. Now the star seems to us to lie in the 
direction in which the telescope points, and a ray which has the 
direction AB appears to have the direction AC. This apparent 
change in the direction of the ray is called its aberration. It is a 
quantity altogether too small to be detected with the naked eye. 

117. Astronomical Refraction. — In §28 we learned that a ray 
of light passing from one medium into another of different density 
is bent out of its course unless it strikes the surface of the new 
medium perpendicularly. 





yy. — Illustration of 
Aberration. 



82 



DESCRIPTIVE ASTRONOMY. 




D E 

Fig. 78. — Refraction. 



In Fig. 78 a ray coming from S through the earth's atmosphere is 
bent from its course. The air may be considered as made up of a 
large number of strata of different densities, the stratum nearest the 
surface of the earth having the greatest density, because it is com- 
pressed by the weight of all the air above it. 
Therefore the ray is being continually bent, 
coming through denser and denser strata, till, 
when it reaches the eye at E, it is coming in 
the direction S'E. The star appears to lie 
at S' instead of S. Since S f lies nearer the 
zenith than S, the effect of refraction is to 
make celestial objects appear nearer the 
zenith, or farther above the horizon, than 
they really are. When a star is in the zenith, 
its rays strike perpendicularly on the atmosphere and suffer no 
deviation. The nearer the horizon a star is, the more its rays are 
refracted. The sun and the moon, when rising or setting, are ele- 
vated by refraction a little over half a degree, so that we see them 
when they are really just below the horizon. 

118. Twilight. — In the air there are not only clouds, but also 
minute particles of dust and globules of water, which reflect the 
sunlight. For some time after 
the sun has set, he still shines 
on the clouds over our heads, 
and on the particles in the 
upper strata of the air. The 
light is reflected from these 
down to the ground, and thus 
produces twilight. When the 
sun gets about 18 below the 
horizon, his rays are shot above 
the clouds and other reflecting 
objects, so that they are no 
longer reflected to us, and twi- 
light ceases. In England and 
other countries of like northern 
latitude, twilight lasts all night in midsummer, because the sun is 
less than 18 below the horizon, even at midnight. 




Fisr. 



79 ■ 



(See Exercise 1.) 



THE EARTH. 83 



EXERCISES. 

119. 1. Suppose the earth to be a perfect and smooth sphere 
and a body to be placed on it, as shown in Fig. 79 ; gravity would 
pull it toward the centre, while it would tend to fly off in the direc- 
tion of the arrow F, because of the earth's rotation. 

(a) As a result of this state of affairs, would the body slide 
toward the equator? 

(b) If the earth were a perfect sphere, would its rotation cause 
the water of the ocean to move toward the equator ? 

(c) Because of the bulge of the earth, is the surface of the water 
at the mouth of the Mississippi River nearer the earth's centre than 
at St. Paul, or farther from it? 

(d) If the earth did not rotate, which way would gravity make 
the river flow? 

(e) Why does the river run south? 

(f) Why does not the water of the ocean leave the poles and 
rush to the equator? 

(g) If the earth were composed entirely of water, and did not 
rotate, what would be its form? 

2. If the earth were a sphere the diameter of which was 7,920 
miles, what would its circumference be according to § 92? Ans. 
24,881.472 miles. 

3. In Fig. 60, if the sun be at F, what points are perihelion and 
aphelion? 

4. In Fig. 60, the sun being at F, prove that the semi-major axis 
(AC) is half the sum of the greatest and least distances (A'F and 
AF) of the earth from the sun. 

5. Why does the earth move most rapidly in its orbit when at 
perihelion? 

6. Cut two equal circles out of stiff paper ; in each cut a narrow 
slit from a point on the circumference to the centre. Then fit the 
circles together in such a way that they will represent the relative 
positions of the planes of the equator and ecliptic. 

7. Let the surface of a small round body, like an apple or orange, 
represent the celestial sphere. On it mark in their proper positions 
the north and south celestial poles, the equator, the ecliptic, the 



84 DESCRIPTIVE ASTRONOMY. 

equinoxes, and the solstices. Also mark the circle in which the 
north celestial pole makes its precessional journey of 25,800 years. 
Does the south celestial pole describe a similar circle on the sphere? 

8. The sun, when at C or D in Fig. 63, ceases to recede from the 
equator and begins to approach it. Show that this fact harmonizes 
with the derivation of the word " solstice." 

9. Why is that equinox in which the sun is on March 20th 
called the vernal equinox? 

10. If the earth's axis lay in the plane of the ecliptic, and always 
pointed directly at the sun, the north pole being toward the sun, in 
what portion of the earth would the sun never be visible ? 

11. If the earth's axis lay in the plane of the ecliptic, but always 
pointed toward one particular fixed star as the earth performed its 
yearly journey around the sun, in what part of the earth would the 
sun never be seen? 

12. If the earth's axis were perpendicular to the plane of the 
ecliptic, and the earth were rotating on its axis and revolving about 
the sun as fast as at present, would all of that part of the earth 
which is in darkness at any instant be in the light twelve hours 
thereafter? 

13. If the earth's axis made an angle of yo° with the plane of 
the ecliptic, and the earth rotated as at present, but did not revolve 
around the sun, is there any part of the earth in which there would 
be no night? 

14. At the moment when the sun lies in the plane of the equator, 
about March 20th or September 22d, does its pull on the equatorial 
protuberance of the earth tend to tip the earth's axis? 

15. When a common almanac makes the statement, "The sun 
enters Aries," does it refer to the sign or to the constellation? 

16. From an almanac which gives the times of rising and setting 
of the sun, find out the number of hours of daylight on the longest 
day of the year, and also on the shortest, at the place for which the 
almanac is computed. 

17. When it is summer in the northern hemisphere, what is the 
season in the southern hemisphere? 

18. At the place where you live does the sun rise very near the 
east point of the horizon in midsummer (June 20th)? Answer the 
same question for Christmas time. 



THE EARTH. 



85 



19. In each of the four figures here given, the earth is repre- 
sented as illuminated by the sun at a certain time of the year. 
Determine the time of year which corresponds to each figure. 





Fig. 80. 



Fig. 81. 





Fig. 82. 



Fis. 



20. If a man were perched on the top of a balloon, which was 
rising straight up through a shower of rain falling vertically, in what 
position would he have to hold the glass tube of § 116 so that the 
raindrops would go straight through it, without touching the sides? 
If the earth be moving directly toward a certain star, would aberra- 



86 DESCRIPTIVE ASTRONOMY. 

tion cause a telescope pointed at a star to deviate a trifle from the 
real direction in which the star lay? 

21. (a) Does refraction make sunrise come earlier than it other- 
wise would, or later? 

(b) What is its effect on the time of sunset? 

(r) Does it lengthen the number of hours of daylight, or shorten 
them? 

22. (a) When the moon is seen near the horizon, which edge 
(or limb, as it is called) is lifted more by refraction, the upper or 
lower ? 

(b) What effect does this have on the apparent shape of the 
moon? 

(c) Does refraction cause the moon to look larger, when near 
the horizon, than when high in the heavens? 

23. At your home does twilight last longer in midsummer, or in 
midwinter? 

24. Denver is in longitude 105 W. of Greenwich, and in north 
latitude 39 40'. Considering the earth as a perfect sphere, what are 
the latitude and longitude of the point on the earth's surface which 
is diametrically opposite Denver? 

25. Two places in England have the same latitude, and differ i° 
in longitude. Two places in Ohio differ i° in latitude, but have the 
same longitude. Are the places in England as many miles apart as 
those in Ohio? 



CELESTIAL MEASUREMENTS. 



87 



CHAPTER VI. 

CELESTIAL MEASUREMENTS. 

" Snatch me to heaven ; thy rolling wonders there, 
World beyond world, in infinite extent, 
Profusely scattered o'er the blue immense, 
Show me : their motions, periods, and their laws, 
Give me to scan." 

Thomson. 



120. Position of Points on a Sphere. — In describing the posi- 
tions of points on the surface of a sphere, a fundamental circle is 
first assumed, bisecting the sphere. In Fig. 84, ABCD is the funda- 
mental circle. Great circles are those whose planes pass through 
the centre of the sphere. All others drawn on the spherical surface 
are small circles. GPH is a 
small circle. Secondary circles 
are great circles which are 
perpendicular to the funda- 
mental circle. All secondaries 
pass through two points called 
the poles of the fundamental 
circle. E and F are the poles 
of ABCD ; AECF and EBFD 
are secondaries. 

The fundamental point is 
a point of reference, which 
lies on the fundamental circle. 
In the figure, A is chosen as 
the fundamental point. 

In finding the position of 
a point on a sphere, we first draw a secondary through the 
point. If P is the point, EPBFD is the secondary drawn. We 
then find out two things: first, how many degrees (measured on 
the secondary) the point is above or below the fundamental circle, 
that is, the length of PB ; second, the number of degrees in that 




Fig. 84. — Circles of Reference. 



88 DESCRIPTIVE ASTRONOMY. 

arc (AB) of the fundamental circle which lies between the funda- 
mental point (A) and the secondary (EPBFD). This system has 
already been employed in defining the latitude and longitude of 
a place on the earth (§ 93). 

121. The Horizon System. — In this system the horizon is the 
fundamental circle. As shown in § 21, the horizon of any place 
is the circle in which the celestial sphere is cut by a plane perpen- 
dicular to a plumb-line suspended at the place. The secondaries, 
being perpendicular to the horizon, are called vertical circles. All 
vertical circles pass through the zenith and nadir (§§ 22, 23). 

The south point of the horizon is usually taken as the fundamental 
point. 

The altitude of a celestial object is the portion of its vertical 
circle lying between it and the horizon. 

Its azimuth is the arc of the horizon, measured from the south 
point around towards the west, to its vertical circle. 

The zenith distance of a celestial object is the portion of its vertical 
circle lying between it and the zenith. In Fig. 84 the altitude of P 
is PB, its zenith distance is EP, and its azimuth (if A be the south 
point of the horizon) is ADCB. 

The celestial meridian of any place is that vertical circle which 
passes through the north and south points of the horizon. It is 
divided by the poles into two equal branches ; the upper branch is 
that in which the zenith lies. 

The prime vertical is that vertical circle which passes through 
the east and west points of the horizon. 

122. The Equator System. — Here the celestial equator is the 
fundamental circle. The secondaries are called hour circles; all 
hour circles pass through the celestial poles. 

The fundamental point is the vernal equinox. 

The declination of a celestial object is the portion of its hour 
circle between it and the celestial equator. When a star is north of 
the equator, it is in north declination ; when south, it is in south 
declination. North declinations are accounted positive ; south, 
negative. 

The right ascension of a celestial object is the arc of the celestial 
equator, measured eastward from the vernal equinox to the hour 
circle on which the star lies. 



CELESTIAL MEASUREMENTS. 



8 9 



Right ascension may be measured in degrees, but is usually 
reckoned in hours, like longitude, 15 being equivalent to one hour. 
The right ascensions and declinations of the fixed stars change very 
little from year to year. 

The north polar distance of a celestial object is the portion of its 
hour circle lying between it and the north celestial pole. 

Instead of the vernal equinox, another fundamental point is fre- 
quently taken, viz. the point where the celestial meridian of the 
place of observation cuts the 
celestial equator. Then the 
hour angle of a celestial object 
is the arc of. the equator em- 
braced between the meridian 
and the hour circle of the 
object. Hour angles are reck- 
oned either east or west of 
the meridian, east hour angles 
being accounted negative : 
they are usually reckoned in 
hours. 

In Fig. 85, NESW is the 
horizon, EQWR the celestial 
equator, PABP' half of the 
hour circle of the star A, and 
V the vernal equinox. AB is the declination of A, VWQB its right 
ascension, and OB its east hour angle. The hour angle of W, the 
west point of the horizon, is QW, which equals +6 h. : that of the 
east point E is QE, which equals — 6h. 

123. Parallax. — The word parallax is a broad term. In general, 
it means the difference in direction of an object when viewed from 
two different standpoints : it is the angle formed by two lines drawn 
from the object to the two standpoints respectively. Thus, the 
parallax of the moon, as seen from Boston and the Cape of Good 
Hope, is the angle between lines drawn from the moon's centre to 
these places. 

Usually the earth's centre is regarded as one of the standpoints : 
then the parallax of Venus, for example, as seen from Denver at 
any instant, is the angle made at Venus by two lines drawn from its 




Fig. 



>5. — The Equator, Horizon, 
Meridian, etc. 



9 o 



DESCRIPTIVE ASTRONOMY. 



centre to Denver and the earth's centre respectively. It is the 
angle DVC in Fig. 86. If the object is in the plane of the hori- 
zon of the place of observation, its parallax is called the horizon- 
tal parallax. DV'C is the horizontal parallax of an object at V. 
If we consider the earth as a perfect sphere, the angle V'DC is a 

right angle. We know 
'V the length of DC, the 

earth's radius. When 
the value of the angle V 
is known, it is easy to 
, compute the distance 
CV by elementary trig- 
onometry. 

If D be a point on the 
earth's equator, CV'D 
is the equatorial hori- 
zontal parallax of an 
object situated at V'. 
The parallaxes of the sun, moon, and planets are large enough to 
be measurable ; but the fixed stars are so distant that the angle 
CV'D is too small to be measured. In getting the parallax of a 
fixed star, the two lines are drawn from the star to the earth and 
sun respectively. Stellar parallax is discussed in § 350. 




Fig. 86. — Parallax. 



TIME. 



124. The Year. — The two principal kinds of years have been 
explained already in § 113. The principles of the Julian and Gre- 
gorian calendars have been set forth in §§ 114 and 115. 

125. Solar Days. — There are two kinds of solar days, apparent 
solar and mea7i solar. An apparent solar day is the interval of time 
which elapses between two successive passages of the sun across the 
upper branch of the celestial meridian of any place. The earth 
rotates on its axis at a uniform rate, so that, if the sun were fixed 
on the celestial sphere, all solar days would be of equal length. 
But the sun appears to creep slowly eastward on the sphere, on 
account of the yearly revolution of the earth, and at an irregular 
rate, creeping more on some days than others, as will be explained 



CELESTIAL MEASUREMENTS. 



91 



in the next section. Hence apparent solar days vary in length, the 
greatest variation being nearly a minute. 

The sun being so irregular a timekeeper, astronomers have 
devised a fictitious sun, called the mean sun, which moves in the 
equator at a uniform rate, completing its apparent journey around 
the celestial sphere in a year. The mean sun crosses the meridian 
sometimes a few minutes in advance of the true sun, at other times 
a few minutes behind it. The greatest difference is sixteen minutes. 
A mean solar day is the interval between two successive passages of 
the mean sun over the upper branch of the celestial meridian of any 
place. 

All ordinary clocks and watches are regulated by mean solar 
time. 

126. Causes of the Unequal Lengths of Apparent Solar Days. — 
There are two principal causes : — 

I. The earth in travelling around the sun does not move at a 
uniform speed. When at perihelion it moves most swiftly; at 
aphelion, with the least velocity. Since the apparent motion of the 
sun eastward is due to the earth's revolution about it, the distance 
over which the sun creeps each 
day on the sphere varies, being 
greatest when the earth's motion 
is most rapid. 

II. But even if the sun moved 
at a uniform rate in the ecliptic, 
apparent solar days would still 
vary in length. Suppose that 
the mean and true suns are to- 
gether at the vernal equinox, 
V, in Fig. 87. After one fourth 
of a year has elapsed, each sun, 
if moving uniformly, will have 
described an arc of 90 ; the true sun will then be at S, the summer 
solstice, and the mean sun at Q, both lying on the same hour 
circle, PSQ. 

Let M be the middle point of VQ, and draw the hour circle PM 
cutting the ecliptic at R. One easily sees that the arc VR is longer 
than VM, so that, when the mean sun arrived at M, the true sun had 




Fig. 87. — Unequal Lengths of 
Apparent Solar Days. 



9 2 



DESCRIPTIVE ASTRONOMY. 



POLE 



not reached R. Hence the mean sun and the true sun, though 
together on the same hour circle at the beginning and at the end of 
their three months' race, did not keep on the same hour circle during 
the race. Now any celestial objects which are on the same hour 
circle cross the meridian at the same time. Hence, while the mean 
sun by its successive passages across any given meridian was mark- 
ing off days of uniform length, those marked off by the true sun 
were not of uniform length. 

127. Another Explanation. — On a sphere, (the larger the better,) 
draw the ecliptic and equator. Mark the summer solstice, and put 
another mark, a quarter of an inch away, on the ecliptic. Draw an 
hour circle through each. Place a mark on the ecliptic a quarter 
of an inch from the vernal equinox and draw hour circles through 
these two points. One sees at once that the first pair of hour 
circles make a larger angle with each other than the second pair. 

_ The quarter of an inch represents 

the amount that the sun creeps on 
the ecliptic in a day. 

At the time of the vernal equi- 
nox an apparent solar day would 
be the time of a rotation of the 
earth plus the time required for it 
to turn through the angle between 
the first pair of hour circles. 

At the time of the summer sol- 
stice an apparent solar day would 
be the time of a rotation of the 
earth plus the time required for it 
to turn through the angle between 
the second pair of hour circles. Since the second angle is larger 
than the first, the two days would be of unequal length. Fig. 88 
exhibits the arcs drawn as directed. 

128. A Sidereal Day. — A sidereal day is the interval between 
two successive passages of the vernal equinox over the upper branch 
of the celestial meridian of any place. Since the apparent daily 
rotation of the celestial sphere is caused by the real rotation of the 
earth, sidereal days are of uniform length. The length of this day 
might be changed by various causes. A gradual shrinkage of the 




ECLIPTIC 



EQUATOR 




Fig. 88. — Variable Motion in 
Hour Angle. 



CELESTIAL MEASUREMENTS. 93 

earth in cooling would make it rotate faster : the friction caused by 
the tidal movement of the water of the ocean tends to impede the 
rotation. However, no change in the length of a day has ever been 
detected by observations. It is considered certain that it has not 
changed a hundredth of a second in the past thirty centuries. 

129. Civil Day and Astronomical Day. — Each of these days con- 
sists of 24 hours of mean solar time. The civil day, employed for 
the ordinary purposes of life, begins at midnight. The astronom- 
ical day begins 12 hours later than the civil day, at noon. 

Thus, Sept. 3, 9 P. M. of civil time is equivalent to Sept. 3, 9 h. 
by astronomical time; but Sept. 3, 9 A. M. of civil time is Sept. 2, 
21 h. by astronomical time. An astronomer when observing at 
night is saved the trouble of changing the date in his record-book 
when midnight comes. A movement is on foot to discontinue the 
use of the astronomical day at the end of the nineteenth century. 

130. Mean Solar and Sidereal Time. — An astronomical clock 
keeping mean solar time at Denver, for instance, reads oh. om. 
o sec, when the mean sun is on the upper branch of the meridian of 
Denver, at noon. Its face is graduated from oh. to 23 h., and the 
hour hand sweeps around the dial once in a mean solar day. Such 
a clock is said to keep " local " mean time. 

A sidereal clock, on the other hand, reads oh. om. osec, when 
the vernal equinox is on the upper branch of the meridian. When 
it reads 10 h. 30m. osec, it shows that the vernal equinox crossed 
the meridian \o\ sidereal hours ago. A reading 23 h. o m. osec 
indicates that the vernal equinox crossed the upper branch of the 
meridian 23 sidereal hours ago, and will cross again in an hour. 

If the mean sun and the vernal equinox were together on the 
celestial meridian of a given place, both the sidereal and the mean 
time clock at that place should indicate oh. o m. osec. After a 
lapse of 24 sidereal hours, the vernal equinox would be on the 
meridian again ; but the mean sun, on account of its eastward 
motion on the sphere, would be east of the vernal equinox, and 
would cross the meridian a little while after it. A mean solar day 
is therefore longer than a sidereal day. As the mean sun travels 
entirely around the sphere in a year, in a day it moves about o-^- of 
the circumference of the sphere, making the mean solar day W-A- h., 
or nearly four minutes, longer than the sidereal day. 



94 DESCRIPTIVE ASTRONOMY. 

131. Right Ascension vs. Sidereal Time. — By the definition of 
right ascension (§ 122), we see that a star whose right ascension 
is 4 h. will cross the meridian of any place 4 h. after the vernal 
equinox has crossed the same meridian. But the sidereal clock 
will read 4 h. We therefore have the following principle. 

The right ascension of any star is equal to the sidereal time at a?iy 
place at the instant when the star is on the meridian of that place. 

Thus when a star whose right ascension is exactly 16 h. is on 
the meridian of the U. S. Naval Observatory at Washington, the 
Washington sidereal clock should read 16 h. O m. o sec. 

When the sidereal time is 13 h., the star mentioned has an east 
hour angle of 3 h., because 3 h. must elapse before it will reach the 
meridian. If the sidereal time be 18 h., the star crossed the meridian 
two hours before, and hence has a west hour angle of 2 h. 

132. Relation between Longitude and Time. — If the longitude of 
a place is one hour west of Greenwich, the mean sun arrives at its 
meridian one hour after it has crossed the Greenwich meridian ; 
hence at noontime at the place in question it is 1 P. M. at Green- 
wich. When a city is two hours east of Greenwich, the sun crosses 
its meridian two hours before it reaches the meridian of Greenwich ; 
when it is 10 A. M. at Greenwich, it is noon at the city. 

133. Where the Date Changes. — A place whose longitude is 
l8o = , or 12 h. west of Greenwich, is also 12 h. east of Greenwich. 
Reckoning it as 12 h. west of Greenwich, its time will be 12 h. less 
than the Greenwich time ; so that when it is 11 A. M. on July 4 
at Greenwich, it will be 11 P. M. on July 3 at this place. But if 
we say that the place is 12 h. east of Greenwich, its time will 
be 12 h. more than the Greenwich time at the same instant, mak- 
ing 1 1 P. M. of July 4. Thus there is a discrepancy of one day, 
according as we count east or west from Greenwich. Mariners 
when crossing the 180th meridian change the date; in going west, 
an entry in the log-book just before crossing the line might be 
dated Wednesday, Sept. 12, 3 P.M. An entry made an hour after- 
ward, if the ship had crossed the line meanwhile, would be dated 
Thursday, Sept. 13, 4 P.M. Similarly, in crossing from the west, 
Tuesday, October 9, would be changed to Monday, October 8. 

134. Standard Time. — There is now in use throughout North 
America a system of standard time, which is a great boon to the 



CELESTIAL MEASUREMENTS. 



95 



business world. Five standard meridians have been adopted, west 
of Greenwich 4, 5, 6, 7, and 8 hours respectively. A city generally 
adopts the time of that standard meridian to which it is nearest. 
The standard times are called respectively Colonial, Eastern, Central, 
Mountain, and Pacific. In a few large cities there is still confusion. At 



Kv^hIT" 



Pittsburg both Eastern and Central times 
are in use. It would be well if all towns 
in the same State kept the same time. 
Several European countries have adopted 
standard times based upon the meridian of 
Greenwich. 

135. Clocks and Watches. — Accurate time 
obtained from various astronomical obser- 
vatories (chiefly from Washington) is tele- 
graphed daily all over the United States. 
The clocks in the observatories are regu- 
lated with the greatest care, so that they 
are rarely over a second in error. The 
pendulum jar of the clock shown in Fig. 89 
is nearly filled with mercury. In warm 
weather the pendulum rod lengthens be- 
cause of the heat; this lengthening would 
make the clock go slow, but the mercury 
expanding rises in the jar, and thus coun- 
teracts the elongation of the rod. In cold 
weather there is a similar compensation : 
such a clock is said to be compensated for 
temperature. 

Good watches should be wound regularly 
and kept in the same position by day and 
by night. When the second hand is at 
sixty, the minute hand should be over some 
minute mark. A sudden change of rate is 
not a sure sign that the regulator needs to 
be moved. The best watches exhibit anomalous variations at times, 
and frequently right themselves without being regulated anew. 

136. The Meridian Circle. — The telescope of this instrument is 
put in the middle of, and at right angles to, an axis which turns in 




Fig. 89. — A Standard 
Clock. 



g6 



DESCRIPTIVE ASTRONOMY. 




Fi gi 90 . _ A Portable Meridian Circle. 



CELESTIAL MEASUREMENTS. 



97 



a bearing at each end as shown in Fig. 90. The axis is horizontal, 
and points due east and west Upon the axis are mounted a couple 
of graduated circles, which are used for finding the altitude of stars 
when they cross the meridian. At the focus of the object-glass near 
the eye-end of the telescope is the " reticle," which, as seen through 
the eyepiece, presents the appearance shown in Fig. 91. It is 
usually made of spider-webs, or of fine lines ruled on a thin piece of 
glass. The " wire " AB is parallel to the axis of the instrument. 





Fig. 91. — A Reticle. 

137. A Star passes the Me- 
ridian. — If the telescope be 
pointed in the direction of a 
star just before it passes the 
meridian, it will be seen to 
move across the field of view 
in a path parallel to AB, cross- 
ing each of the five parallel 
wires. The reason for this is 
shown in Fig. 92. A line drawn 
from the star at S through the 
centre of the object-glass meets 
the star's image at I. As the 
star moves to S' and S" its 
image moves to V and I" '. Therefore, the observer sees the star's 
image crossing the wires of the reticle. 

If ST' is perpendicular to the rotation axis, it lies in the plane 

7 



Fig. 92. — Motion of a Star's Image. 



98 DESCRIPTIVE ASTRONOMY. 

of the meridian, for the meridian plane is perpendicular to a hori- 
zontal east and west line. Therefore both the star S' and its image 
r lie in the plane of the meridian. The reticle is so placed that 
the star's image when at V lies on the middle wire in Fig. 91. 

138. Determination of Clock Error. — In the Nautical Almanac, 1 
which is issued yearly, is to be found an extensive list of bright 
stars, the right ascensions of which are given. The right ascension 
of any star, as we have learned, equals the sidereal time when that 
star crosses the observer's meridian. The astronomer who wishes 
to find the error of his sidereal clock selects from the list a star, 
Sirius, for example, and points the telescope of the meridian circle 
in such a direction that Sirius when it crosses the meridian will 
pass through the field of view. As it crosses each wire of the 
reticle he notes the reading of the clock as below: — 





6 


39 


46.3 






39 


56.2 






40 


1 1.4 






40 


26.6 






40 


3 6.6 


Average, 


6 


40 


11.42 



The average of these readings is a pretty accurate value of the 
clock reading when the star crossed the middle wire, which repre- 
sents the meridian. Turning to the almanac, he finds that on the 
date of observation the right ascension of Sirius was 6 h. 40 m. 
26.94 sec. Hence, if the instrument was in perfect adjustment, the 
error of the clock was 15.52 sec. Was the clock fast or slow ? 

To attain greater accuracy, the astronomer observes a number 
of stars. Four stars, after allowance had been made for errors in 
the position of the meridian circle, might give him for the clock 
error respectively 15.52 sec, 15.46 sec, 15.53 sec, and 15.45 sec. The 
average of these is 15.49 sec. 

By comparing observations made on two dates, the amount that 
a clock gains or loses in a day can be found. This amount is the 

1 A copy of this work, if not obtained through a Senator or Representative, may be 
had by sending one dollar to the Nautical Almanac office, Washington, D. C. The 
British, German, French, and some other nations publish such almanacs. 



CELESTIAL MEASUREMENTS. 



99 



daily rate. Sidereal time is easily reduced to mean time by tables 
given in the Nautical Almanac. 

139. The Chronograph. — A chronograph is employed to facilitate 
noting the time when any phenomenon occurs. Its most common 
use is to record the reading of a timepiece at the instant when an 
astronomer sees a star cross a wire in the reticle of his meridian 
circle. 




Fig. 93. — A Chronograph. 



A sheet of paper is wrapped around the barrel, which is rotated 
once a minute by the mechanism, driven by a weight not shown in 
the figure. If this were the only motion, the pen which rests on 
the barrel would draw a circle around the cylinder in a minute, and 
repeat the same circle the next minute, and so on, as long as the 
mechanism ran. But the carriage which holds the pen is mounted 
on a long screw, shown in front of the barrel : the screw rotates once 
a minute, and continually moves the pen carriage, which is set near 
one end of the barrel at starting, toward the other end of the barrel. 
Consequently the pen, instead of making a single circle over and 
over, makes a long spiral line, like a screw thread, running from 
one end of the barrel to the other. 



IOO 



DESCRIPTIVE ASTRONOMY. 



140. Records of a Clock and Key. — By suitable electrical connec- 
tions, which cannot well be explained here, a clock causes the pen 
to give a quick vibration each second, so that a series of notches 
are made in the line, as shown in Fig. 94. No notch is made at the 



-Zt^-V 



/6 IS i+ /S fZ. // to 9 8 7 6 & <+ 3 2. 

Fig. 94. — A Chronographic Record. 



n 



fifty-ninth second of each minute : the first notch thereafter marks 
the beginning of a new minute. The observer notes the time which 
the clock read when some particular notch was made, and marks that 
notch. In Fig. 94 the notch for 9 h. 6 m. osec. was thus marked: 
from this record he can tell the time when any other notch was made 
by the clock. 

The astronomer, when observing, has at his side a telegraph key, 
connected with the chronograph : when he wishes to note the time, 

he presses the key quickly, and the 
pen makes a notch at that instant. 
Fig. 94 shows some records made 
between 9 h. 1 m. and 9 h. 6 m., on a 
small portion of a sheet. In the 
place where the fifty-ninth second of 
each minute is omitted, the observer 
has written on the lines the numbers 
of the successive minutes. The 
numbers of the seconds of each min- 
ute are written along one of the 
lines. There are three extra marks, 
made when the observer pressed his 
key. One of the marks was made 
at 9 h. 1 m. 4.5 sec, another at 9 h. 

3 m. 9.4 sec, and the third at 9h. 

4 m. 13.8 sec 

141. Determination of Latitude. — Let D in Fig. 95 be a point on 
the earth's surface, NS its horizon, P'P" the earth's axis, and EQ 




Fig- 95- 



CELESTIAL MEASUREMENTS. IOI 

the equator. ZDO is perpendicular to NS. PD is parallel to P'P", 
and therefore points to the north pole of the celestial sphere. DT 
is parallel to EQ. PDT is a right angle. By definition (§ 94) 
the latitude of D is DOE. This equals ZDT = 90 - ZDP=PDN, 
which by definition (§ 121) equals the altitude of the celestial 
pole. Hence the latitude of the place of observation equals the alti- 
tude of the pole. Though an astronomer cannot see the north celes- 
tial pole, he can find its altitude by observing the Pole Star when it is 
on the meridian. In Fig. 96, D is the place of observation, NS the 
horizon, NZS the meridian, P the north celestial pole, Y and P" the 
two positions of the Pole Star when it crosses the meridian. By 




Fig. 96. — Latitude found by Observation of Polaris. 

means of the divided circle on the axis of the meridian circle, the 
astronomer measures the angle P'DN, the altitude of the Pole Star 
when it is on the meridian above the pole. Twelve hours later he 
measures P"DN. 

P'DN = PDN + PDF 
P'DN = PDN - PDP" 



P'DN + P"DN = 2 PDN + PDF - PDF'. 

But PDP' = PDP". 

Therefore FDN + P"DN = 2 PDN 

PDN = i (FDN + P"DN). 
PDN is the latitude required. 

The method by which the angles P'DN and P"DN are measured 
will be understood readily by examining an engineer's transit, such 
as is shown in Fig. 97. 



102 



DESCRIPTIVE ASTRONOMY 




Fig. 97. — An Engineer's Transit. 



CELESTIAL MEASUREMENTS. 



OS 



142. Determination of Longitude. — We have seen (§ 132) that 
the difference in longitude between two places, such as Washington 
and Chicago, is equal to the difference at any instant between the 
readings of two clocks, one of which keeps correct Washington time, 
while the other keeps Chicago time. If a chronometer keeping 
Washington time be carried to Chicago and compared with the 
Chicago clock, the difference between their readings, if both are 




Fig. 



A Chronometer. 



correct, is the difference of longitude sought. In practice several 
chronometers are used, the errors of which — for no chronome- 
ter or clock runs exactly right — are very carefully determined 
by observations of the stars. The electric telegraph, however, 
furnishes a much more accurate method. By quite simple mech- 
anism the Washington clock, as it ticks, makes a telegraphic 
sounder at Chicago click, so that an astronomer at Chicago can 



104 



DESCRIPTIVE ASTRONOMY. 



compare the telegraphic beats of the Washington clock with his 
own. The times at which the signals are to be sent are agreed 
upon beforehand. 

143. The Position of a Ship. — A mariner usually finds the lati- 
tude and longitude of his ship by observations of the sun made 
with a sextant, a little instrument easily held in the hand. A 
chronometer keeping some standard time, as that of Greenwich, 
is also necessary. In Fig. 96, let M represent the sun when on 
the meridian. The mariner measures its altitude, MS, with the 




Fig. 99. — A Sextant. 



sextant. From the almanac he finds PM, the distance of the sun 
from the pole. The sum of these gives PMS, which, subtracted 
from 180 , leaves PM, the latitude desired. 

To find the longitude, the altitude of the sun is measured about 
the middle of the forenoon or afternoon, and the reading of the 
chronometer, keeping Greenwich time, is noted at the same time. 
Suppose that the measured altitude was 62 15' 20." By means of 
astronomical tables the mariner computes the time at which the 
sun had that altitude; it might have been 3 h. 16 m. 27 sec. If 



CELESTIAL MEASUREMENTS. IO5 

the Greenwich time, found from the chronometer, 1 was 4I1. 29 m. 
48 sec, the ship was evidently in longitude 1 h. 13 m. 21 sec. west 
of Greenwich. 



EXERCISES. 

144. 1. Is the arctic circle on the earth a great circle or a small 
circle? 

2. Consider the earth as a perfect sphere, and the equator as the 
fundamental circle ; what is the name applied in geography to the 
secondaries? 

3. Consider the earth as a perfect sphere. In estimating the 
longitudes of points from Greenwich, what point is taken as the 
fundamental point? 

4. (a) When the sun is just rising, what is its altitude? 
(b) When a star is in the zenith, what is its altitude? 

5. (a) What is the azimuth of a point on the prime vertical, 
the point being west of the zenith? 

(b) What is the azimuth of the north celestial pole? 

(c) What is the azimuth of the east point of the horizon? 

6. (a) Does the celestial meridian of any place pass through the 
nadir? 

(#) Do the celestial poles lie on this meridian? 
(V) Is there any point on the earth where the meridian coincides 
with the celestial equator? 

(d) Does your celestial meridian cut the celestial equator? 

(/) If so, can you point your finger toward a point of inter- 
section? 

(/) Where does the prime vertical cut the meridian? 

(<£") What position has the plane of the prime vertical with refer- 
ence to that of the meridian? 

7. (a) The declination of a star is +20° ; what is its north polar 
distance? 

1 The reading of the chronometer face is not the true Greenwich time. Before 
leaving port the error of the chronometer (usually a few seconds) and the daily rate 
(§ 138) were found by astronomical observations. From these the error of the chro- 
nometer at any time during the voyage can be computed, and allowance made for it to 
get the true Greenwich time. While chronometers do not keep exactly the same rate 
from week to week, they run closely enough for the practical purposes of navigation. 



I06 DESCRIPTIVE ASTRONOMY. 

(b) What is the north polar distance of a star whose declination 
is -30 43'? 

(c) What is the right ascension of the autumnal equinox? 

(d) What is the sun's right ascension, when it is in the summer 
solstice? 

(e) What is its right ascension when in the winter solstice? 

(/) If the vernal equinox be on the meridian of Chicago, what 
is the right ascension of a star which is rising at that instant at the 
east point of the Chicago horizon? 

(g) At the same instant as above a star is setting at the west 
point of the horizon; what is its right ascension? 

8. {a) If a star having a right ascension of 18 hours is now on 
the meridian, what is the right ascension of a star which now has an 
east hour angle of 3 hours? 

(b) When a star the right ascension of which is 8 hours is on 
the meridian, what is the right ascension of a star which has a west 
hour angle of 5 hours? 

(c) May a number of different stars have the same right 
ascension? 

(d) May a number of stars have the same declination? 

(e) Is it possible for two stars to lie on the same hour circle, and 
have different right ascensions? 

(/) If the right ascension of a fixed star now is 1 1 h. 28 m., 
what is it 3 hours hence? 

9. Qa) When a correct sidereal clock reads 17 h., the hour angle 
of a certain star is 5 h. west. What is the star's right ascension? 

(#) The sidereal time being 17 h. 26 m., a star is found to have 
an east hour angle of 4I1. ; what is the star's right ascension? 

10. Reduce March 9, 7 A. M., civil time, to astronomical time. 

11. The sun and a star are in the vernal equinox, and both are 
setting below the horizon of some place. 

(«) A month afterwards, which will set the earlier? 
(#) Which will rise the earlier? 

(c) On July 20th, a certain star sets below the horizon of Boston 
at 8 P. M. A month afterwards, will it set at about the same time ? 

12. By use of the telegraph it was found that when a Washing- 
ton clock read 9 h. o m. o sec, a St. Louis clock read 8 h. 7 m. 
22.93 sec. The Washington clock was 26.37 sec. f ast > and the St. 



CELESTIAL MEASUREMENTS. IOJ 

Louis clock was 15.92 sec. slow. What is the difference of longi- 
tude between the two places? 

13. An astronomer noted the following readings of his sidereal 
clock when a certain star crossed the wires in the reticle of his 
meridian circle. 



h. 


m. 


sec. 


3 


14 


17.2 




14 


3O.6 




14 


44.O 




14 


57-4 




J 5 


10.7 



The right ascension of the star was 13 h. 14 m. 3 1.68 sec. If 
the meridian circle was in perfect adjustment, what was the clock 
error? 

14. When Polaris is on the meridian above the pole, an observer 
measures its altitude, finding it to be 39 46'. Twelve hours later 
its altitude is 37 18'. What is the latitude of the place of 
observation? 

15. (a) If the altitude of a star is 16 , what is its zenith 
distance? 

(#) If the declination of a star is +34° 5', what is its north polar 
distance? 

(V) The north polar distance of a star is 11 6° 35'. W T hat is its 
declination? 

16. A mariner measures the altitude of the sun at noon, getting 
6g° 47' 25" as his result. From the almanac he finds that the 
sun's declination at the time when the altitude was measured was 
—6° 37' 49". Find the latitude of the ship's position. 

17. At midnight on July 25th, a chronometer was 38.92 sec. 
slow. Its daily rate was 0.84 sec. gaining. Assuming that it kept 
this rate, what was its error at noon of July 31st? 

18. The captain of a vessel measures the altitude of the sun on 
the afternoon of May 16th; his chronometer (keeping Greenwich 
time) reads 4 h. 16 m. 29 sec. when the altitude is measured. By 
means of data given in the almanac, he computes that the local 
time when the sun had the measured altitude was 3 h. 27 m. 58 sec. 
What was the longitude of the ship at the time of the observation? 



I08 DESCRIPTIVE ASTRONOMY. 



CHAPTER VII. 

THE MOON AND ECLIPSES. 

" In full-orbed glory yonder moon divine 
Rolls through the dark blue depths." 

Southey. 

145. Distance, Diameter, Orbit, Nodes. — The moon, though the 
most conspicuous of all the heavenly bodies except the sun, is 
really quite small, being only 2,163 miles in diameter. 

Its average distance from the earth's centre is 238,840 miles. As 
the earth journeys around the sun in an ellipse, so the moon travels 
around the earth in a similar orbit, the earth being at one of the 
foci of the ellipse. The orbit is nearly a circle, and is inclined to 
the plane of the ecliptic only 5 . The moon appears to us to de- 
scribe a circle on the face of 
the sky every month, the circle 
being roughly coincident with 
the ecliptic. The moon's path 
intersects the ecliptic at two 
opposite points, called the 
moon's nodes. 

146. Periods, Sidereal and 

Fig. 100. — Orbits of the Earth and Moon. « ,. -p, • ■, , , • 7 

5 Synodic. — The sidereal period 

of the moon is the time required for making one revolution about 
the earth. In Fig. 101 the sun, the earth, and the moon are in line, 
in the positions S, E, and M. The earth and the moon pursue their 
appointed paths until they arrive at E' and M' respectively, the line 
E'M' being parallel to EM. The moon has now accomplished a 
complete revolution ; the time required is nearly 27J- days, which is 
therefore the sidereal period. But the moon is not yet opposite the 
sun, as it was at the start. When it reaches M", the earth being at 
E", it is opposite the sun again, and a synodic revolution has been 
accomplished. The synodic period is over 29 \ days. 




THE MOON AND ECLIPSES. 109 

147. Time of Crossing the Meridian. — Since the moon moves rap- 
idly eastward among the stars, it does not cross the meridian of 
the observer at the same time every day, but crosses about 5 1 min- 
utes later on the average each day than the preceding. The amount 
of daily retardation varies considerably from causes analogous to 
those which cause the sun to be an irregular timekeeper. (§ 126.) 





Fig. 102. — Illustration of the 
Fig. 101. — Sidereal and Synodic Periods. Moon's Rotation. 

148. Rotation. — The moon always presents the same face to us : 
an unthinking person might conclude from this that it did not rotate 
at all. Let a boy trace a circle on the ground around a tree, and 
station himself south of the tree and facing it; he then faces north. 
Let him walk around the circle, continually facing the tree. At S 
in Fig. 102, he faces north, at E west, at N south, and at W east. 
When he arrives at S again, he has turned completely around once. 

149. Librations. — The moon rotates on its axis at a uniform rate, 
but since it does not move with uniform rapidity in its orbit, we 
sometimes see a short distance around one edge or the other. In 
Fig. 103, when the moon is at M, we see the portion ACB. When 
moving more swiftly than its average, it will describe 90 of its 
orbit and arrive at M' in a little less than one fourth of its sidereal 
period. So it will not have rotated one fourth of a complete turn, 
and an observer at the earth, though not able to see the point A, 
will look past B, as shown by the figure. 



I IO 



DESCRIPTIVE ASTRONOMY. 



Furthermore, the moon does not stand quite upright on its orbit, 
that is, its axis is not perpendicular to the plane of its orbit Hence, 
as shown in Fig. 104, we sometimes see past the north pole and 
sometimes past the south pole. 

These apparent oscillations are called librations. There is a 
minute libration due to the fact that we are not at the earth's centre. 





EARTH 



Fig. 103. — Libration. 



Fig. 104. — Libration. 



When the moon is rising, we can see farther over its upper edge 
than a man whose eye is at the earth's centre. Fifty-nine per cent 
of the moon's surface has been seen by astronomers. 

150. Phases of the Moon. — The moon shines by reflecting the 
sunlight which strikes it. When the moon is between us and the 
sun, its illuminated side being toward the sun, we see the dark side. 
The moon is then said to be new. A week later, when it has moved 
from A to B (Fig. 105), half of the illuminated hemisphere is visible 
to us, and the moon is said to be at its first quarter. After the 
lapse of another week it is at C, opposite the sun, and the whole of 
the illuminated hemisphere can be seen by us ; this phase is full 
moon. A week thereafter, when the moon has reached D, it is in 
its last quarter, half the bright hemisphere being visible. For a 
week before and after new moon, when but a small part of the moon 
looks bright to us, it is crescent. During the week preceding full 
moon, and also during the following week, when more than one half 
and less than the whole of the bright part of the moon is turned 



THE MOON AND ECLIPSES. 



I I I 



toward us, the moon is gibboits. The appearances of the moon are 
shown in Fig. 106. 

151. Earth Shine. — When the 
moon is crescent, one easily sees 
the dark portion of it, as well as 
the brilliant crescent of light. 
The dark part is bounded on the 
side next to the sky by a narrow 
rim of silvery light, so that the 
whole looks not unlike a cake 
basket hung in the sky : it is 
popularly called " the old moon 
in the new moon's arms." Why 
is the dark part seen so easily? 
Some of the sunlight which falls 
upon the earth is reflected away, 
and, striking upon the side of the 
moon turned toward us, illumi- 
nates it sufficiently to render the 
dark part visible to us. 

152. Occupations. — The moon 
in its monthly round passes be- 
tween us and many of the stars, hiding them from view so that they 
are occulted (hidden) for a time. The occultation of bright stars 




— The Moon Illuminated. 




NEW 




FULL 




GIBBOU5 
WANING 



LAST 
QUARTER 



CRESCENT 
WANING 



Fig. 106. — The Moon's Phases. 

can be observed with the naked eye. The fainter ones are blotted 



112 



DESCRIPTIVE ASTRONOMY. 



out by the moon's brilliancy as it approaches them. At the instant 
when the limb of the moon gets into line between the star and the 
observer's eye, the star vanishes as if annihilated. After an hour or 
less, the star reappears on the opposite side of the moon. Both the 
disappearance and the reappearance are instantaneous. Observa- 
tions of occultations are sometimes made by mariners to find the 




Fig. 107. — The Moon: Photographed at the Lick Observatory. 



errors of their chronometers : from the data given in the Nautical 
Almanac the Greenwich time of the occurrence of an occultation of 
a given star, as seen at a given place, is computed. The chronom- 
eter reading is noted at the time of the star's disappearance ; by 
comparing this reading with the computed time the error of the 
timepiece is found. 



THE MOON AND ECLIPSES. 113 

153. Appearance to the Naked Eye. — To the naked eye, the face 
of the full moon appears to be diversified with irregularly shaped 
dark spots. Most people see a strong resemblance to a human 
face. Many perceive a complete human figure, said among the 
French to be Judas Iscariot, transported thither as a punishment. 
Humboldt states that it is a popular belief among the people of 
Asia Minor that the moon is a mirror, which reflects back the 
image of the earth. When one examines the moon with an opera- 
glass, the dark spots are seen to be the smoother portions of the 
moon's surface. They are simply vast plains : on the maps they 
are designated as seas, being thought by the early lunar cartogra- 
phers to be such. 

154. Use of the Telescope. — It is well at first to put on a low 
magnifying power, so that the whole of the moon may be in the 
field of view at once. At the time of full moon the view is very 
much less satisfactory than at the first quarter (Fig. 107), and for 
three days thereafter. For at the time of full moon the shadows 
of the mountains on the moon are invisible to us, because they 
are cast directly behind them, and we lose the effect of contrast 
between the objects and their shadows. At the time of the first 
quarter, those mountains which are near the terminator (boundary 
between the illuminated and unilluminated portions of the moon) 
cast magnificent shadows ; the rugged details of these mountainous 
forms are then very conspicuous in the telescope (Fig. 113). 
After viewing the entire lunar disk with a low power, higher 
powers may be tried with advantage on the more conspicuous 
objects. They bring out a wealth of detail, which wellnigh baffles 
delineation. 

No telescope ever yet constructed can bear with advantage, even 
on the finest night, a power exceeding 3,000 diameters : such a 
power would bring the moon within 80 miles. 

Objects as large as the largest buildings on the earth might be 
perceived, if they differed considerably in color from the back- 
ground upon which they were seen. 

155. Lunar Topography. — The features of the landscape maybe 
divided into the following classes : plains, craters, mountain peaks. 
mountain ranges, rills, clefts, and rays. Some hundreds oi these 
objects have received names : a few of the most prominent ones 

8 



H4 



DESCRIPTIVE ASTRONOMY. 



are shown on the accompanying skeleton map (Fig. 108), in which 
the moon is represented as seen in an inverting telescope. The 
numbered craters have the following names : — 



i. Clavius. 

2. Schiller. 

3. Schickard. 

4. Tycho. 

5. Catharina. 



6. Cyrillus. 

7. Theophilus. 

8. Arzachael. 

9. Alphonsus. 
10. Ptolemv. 



11. Gassendi. 

12. Maskelyne. 

13. Copernicus. 

14. Kepler. 

15. Eratosthenes. 



16. Archimedes. 

17. Burg. 

18. Aristotle. 

19. Plato. 




Fig. 108. — Skeleton Map of the Moon. 



156. The Plains. — These are, as before remarked, darker than 
the rest of the surface, and smoother. With low powers they look 
much as if they were dry beds of ancient seas. Under a high 
power many minute pits are discovered besprinkling the plains ; 



THE MOON AND ECLIPSES. I 1 5 

the surface is found to be really quite rough and wrinkled. The 
boundaries of these " seas," as they are denominated on the map, 
are not- always sharply defined ; in some cases, the bounding 
" sea-wall " is nearly complete, and composed in part of pre- 
cipitous cliffs, which exceed in grandeur any similar terrestrial 
formations. 



Fig. 109. — Conspicuous Craters. (Nasmyth and Carpenter.) 

157. Craters. — Even with an opera-glass one may see that the 
brighter portions of the moon's surface are thickly bestrewn with 
irregular ring-shaped mountains. Kepler 1 conjectured that these 

1 The great astronomer, 1 571-1630, who discovered that the orbits of the planets 
are ellipses, and formulated certain famous laws concerning their motion. 



n6 



DESCRIPTIVE ASTRONOMY 



were pits dug by the inhabitants of the moon to shelter themselves 
from the sun during the long lunar day. 

Had he known that a large number of these pits are over fifty 
miles in diameter, and that the walls of some of them are three or 
four miles high, he would hardly have broached this theory. The 
craters present a wonderful diversity of size and aspect. Thou- 





Wt&'^fc ^ 







Fig. no. — The Crater Copernicus. (Nasmyth and Carpenter.) 



sands are so minute that in the most powerful telescope they look 
like mere pin-pricks, being half a mile or less in diameter. The 
largest ones are over one hundred miles across, and are perhaps 
more properly called walled plains, especially if the floor of the 
crater is quite smooth. In the centre of a crater a mountain or 
a small group of peaks is usually found. 



THE MOON AND ECLIPSES. 



117 



The interiors of most of these craters are lower than the general 
surrounding level, but in some cases the interiors are elevated 
above the general surface. Some are isolated : others are crowded 
so thickly together that they overlap. Some walls are very pre- 
cipitous ; others are a series of magnificent terraces. The im- 
mensity of some of these formations is realized by looking at the 
cut of Copernicus (Fig. no). The larger of the craterlets around 
it are as big as Vesuvius. In the figure, these, as well as the other 
features to be described, are easily distinguished. 




Fi£. in. — Copernicus. 



158. Nature and Cause of the Craters. — The craters are frequently 
referred to as the moon's volcanoes. It would be a mistake to 
infer from this that there are any signs of present volcanic activity 
on the moon. The entire appearance points to the theory that 
the moon was once a molten mass, and that by its cooling and 
solidification its various topographical features were formed. 

Similar appearances are to be found upon the earth; Fig. 112 
shows the marked similarity of the neighborhood of Vesuvius to 
a portion of the moon. On the cooling tap cinder from the fur- 
naces for the production of iron there is formed a thin crust, 
which is soon broken open in spots by the pressure of the con- 



u8 



DESCRIPTIVE ASTRONOMY. 



fined gases ; the molten matter exudes through the holes and 
cracks, and frequently forms miniature volcanic cones. The cen- 
tral mountains in lunar craters were probably formed by the last 
sluggish oozings from the heated interior. 

159. Mountains and Mountain Ranges. — Isolated mountain peaks 
are comparatively rare, though occasionally one can be found 
rising to a height of a mile or more out of a comparatively smooth 
landscape. There are a few mountain chains, the most prominent 




Fij 



The Terrestrial Crater Vesuvius. (Nasmyth and Carpenter.) 



of which is named the Apennines ; it is 450 miles long, and bristles 
with peaks which rival the Andes in altitude, and cast magnificent 
shadows nearly 100 miles long athwart the neighboring plains. 
By measurement of these shadows the heights of many peaks have 
been determined. On one side, as shown in Fig. 113, they rise 
gradually from the plain, but on the other they are terminated by 
dizzy precipices, some of which are over three miles high. 

160. Rills, Clefts, and Rays. — Rills are narrow, deep, and tor- 
tuous valleys, which look like the beds of dried up streams. 



THE MOON AND ECLIPSES. 



119 



Clefts are narrow rifts of great depth. Two fine ones are shown 
in Fig. 113, starting from opposite sides of the largest crater: each 
is over 100 miles in length ; near the centre it is a mile in width. 
They are thought to be not less than ten miles in depth, and 
must be appalling in grandeur to a lunar traveller. 




Fk 



The Lunar Apennines. (Nasmyth and Carpenter.) 



Rays are streaks which diverge in all directions from some of 
the craters. They are best seen at the time of full moon. The 
finest system radiates from Tycho, which, with an opera-glass, 
looks as if it were a pole of the moon, the rays being meridians 
diverging from it. Copernicus has a smaller system, shown in Fig. 
109. The rays are on the general surface, being neither elevated 



120 



DESCRIPTIVE ASTRONOMY. 



above nor depressed below it; they go over crater walls and 
through valleys, just as if some one had painted them there with 
a gigantic brush after the landscape had assumed its present form. 
No satisfactory explanation of these has been given. Possibly 




Fig. 114. 



The Crater Vendelixus : Drawn from a Lick Photograph. 



they are discolorations of the surface by vapors rising from cracks 
too narrow to be visible to us. 

161. Changes. — There has been considerable discussion among 
astronomers as to whether any changes have ever been noted in the 



THE MOON AND ECLIPSES. I 2 I 

lunar topography. A given crater may change its aspect in an 
hour, because of the shifting of its shadow; such changes are most 
noticeable when the sun is just rising or setting at the crater (Fig. 
114). For this reason, a careful examination, extending over 
several nights, is necessary to enable one to gain a correct idea of 
the details of the form of any crater or mountain. When we add 
to this the usual blurring caused by the unsteadiness of our atmos- 
phere, and the minute errors which the most skilful draughtsmen 
are liable to make in their delineations, we can see how easy it is 
to imagine slight changes where none have really taken place. 
There is not the slightest evidence that any eruptive forces are at 
work. Possibly a very few land-slips have occurred. Lunar pho- 
tography may after some years give us decisive results. 

162. Atmosphere. — It has been demonstrated that the atmosphere, 
if it exists, is extremely rare, the pressure not exceeding a thou- 
sandth of that at the earth's surface. When a star is occulted, it 
ought, if there were a lunar atmosphere one tenth as dense as that 
of the earth, to suffer a change of brightness and color, when close 
to the moon's limb ; further, as one sees the sun after it has really 
set, on account of refraction, so the time of the star's disappearance 
would be much retarded by the refraction of the lunar atmosphere, 
and its reappearance would be accelerated. 

Twilight causes an illumination of the terrestrial landscape for 
some time after the sun has set. No marked illumination of this 
sort has been seen at any point on the moon. 

The lunar spectrum is identical with the solar ; this shows that 
the sun's rays when reflected from the moon suffer no noticeable 
absorption by its atmosphere. 

163. Water. — What may be on the side of the moon which we 
never see, we cannot affirm, but there is no reason to think it 
different from the face presented to us. Any lake covering as much 
as a square mile, if not hidden from view by some obstruction, 
would have been discovered ere this. 

On account of the coldness of the lunar days, as well as nights, it 
is not unlikely that water, if present, would exist only in a frozen 
state. There are no indications of either ice or snow. 

164. The Water and Air Formerly. — If the moon was once, as is 
generally supposed, a portion of the earth, it must have carried 



122 DESCRIPTIVE ASTRONOMY. 

some water and air with it when they separated, though the earth 
kept the lion's share, on account of its stronger power of attraction. 
What has become of the water we can only conjecture : great cav- 
erns may have been formed in the process of cooling, into which 
both the air and water have sunk. The water may have become 
chemically united with the molten rock in the process of crystal- 
lization. A rock when heated expels gases formerly absorbed ; 
in cooling slowly it can absorb them again ; perchance the lunar 
atmosphere was absorbed in this way. Still another theory is 
based on the exceedingly swift motion of the molecules of gases. 
The force of gravity at the moon's surface is but one sixth of that 
at the earth's, so that a rifle bullet there would " carry" IOO miles. 
Some have thought that the molecules of the lunar atmosphere 
may have escaped from this feeble attraction and gone off into 
space, never to be recovered ; but this theory will not stand a critical 
examination. 

165. Light and Heat reflected to the Earth. — Five sixths of the 
sunlight which falls upon the moon is absorbed, the rest being 
reflected away. The sun gives 600,000 times as much light as the 
full moon ; yet the full moon in mid-heaven gives sufficient light to 
enable one to read this page. The measurement of the heat sent 
us by the moon is difficult, because its amount is " vanishingly 
small." If the full moon could shine upon us steadily for a year, 
it would give us as much heat as the sun does in three minutes. 

166. Temperature at the Moon. — During the long lunar day the 
sun blazes upon the moon's plains with a fury unmitigated by a 
protecting atmosphere, and untempered by the presence of clouds ; 
yet the plains are cold. The air of our own planet acts as a blanket 
to keep us warm. The solar rays pierce the atmosphere readily 
and find lodgment in the earth ; but when the earth strives to radi- 
ate its heat back into space, the air checks the radiation. On lofty 
mountain tops, over which there is a much thinner air blanket than at 
their base, the rigors of eternal winter reign. The lunar atmosphere 
is entirely inadequate to check radiation, so that under direct sun- 
shine the temperature of the moon's surface probably never rises 
above the freezing point of water. During the lunar night the tem- 
perature is believed to be no higher than 200 below zero on the 
Fahrenheit scale. 



THE MOON AND ECLIPSES. 1 23 

167. Life on the Moon. — Enough has been said to show that there 
is no such animal and vegetable life on the moon as on the earth. 
It is a land of death. The sky is a pall of black, studded with 
stars by day as well as by night. The rising sun, unheralded by the 
beautiful sky tints which accompany the dawn on earth, darts his 
garish beams athwart the desolate landscape, causing the lofty peaks 
to cast long shadows which vie with the sky in blackness. No bird 
song greets him ; there is no rustle of a breeze, or plash of a brook, 
or murmur of an ocean. Should " lips quiver and tongues essay to 
speak," no sound from them would break the eternal silence. Dark 
pits innumerable yawn on every hand. The silvery rims of mighty 
craters encircle abysses of darkness. As the sun slowly rises in the 
sky, the fierce chill of the departing night is slowly mitigated ; but 
no manlike being welcomes returning warmth. 

The earth hangs continually in mid-heaven, waxing from crescent 
to full and waning again, swiftly spinning on its axis and bringing 
into view an ever shifting panorama of cloud and continent and 
ocean. No star forgets to shine ; the weird glory of the solar 
corona and the fantastic forms of the protuberances can be seen in 
all their beauty by screening off the direct light of the sun. The 
Milky Way girdles the sky, bejewelled with thousands of glittering 
orbs. The eye is enchanted by the glories above, though the mind 
shrinks from contemplation of the desolation all about. After four- 
teen terrestrial days have elapsed, the long shadows stretch them- 
selves eastward, the sun slowly sinks beneath the western horizon, 
and night with its terrible rigors of cold comes on apace. Such is a 
lunar day. 

168. The Moon and the Weather. — Various fanciful notions con- 
cerning the moon's influence upon the weather are rife among igno- 
rant persons. One hears of wet and dry moons ; when the cusps 
of the new moon have a decided upward slant, fair weather is said 
to be presaged ; when they do not slant upwards, foul weather is 
to be expected. Such ideas are arrant nonsense. The positions 
of the moon's cusps can be foretold for thousands of years ; the 
weather, not for a single week. The connection of changes of the 
weather with changes of the moon's phases is likewise unfounded. 
Since the moon changes its phase every week, all weather changes 
must occur within four days of some change of lunar phase. We 



124 DESCRIPTIVE ASTRONOMY. 

know of no ways in which the moon would affect the weather except 
by its heat, or by raising aerial tides, or by disturbances of the 
magnetic conditions. 

Its heat is almost immeasurably small ; the effect of aerial tides 
on the readings of the barometer is insignificant. Certain minute 
magnetic disturbances have been detected, which seem to be de- 
pendent upon the varying distance of the moon. The idea that the 
full moon clears away clouds probably has its foundation in the fact 
that the moon renders the rifts in a lightly clouded sky conspicuous, 
while they would otherwise escape notice. 

169. The Moon's Worth to Man. — The most stupendous work done 
by the moon for man is the rise of the tides, of which it is the chief 
cause. The flood tide lifts ponderous ships over dangerous bars at 
the entrances of harbors. Merchantmen are carried from the mouth 
of the Thames up the river to the busy wharves of London on the 
bosom of the tide. The tides scour the mouths of rivers, carrying 
away the pestilence breeding matter which tends to accumulate 
there. 

The enormous power of the tides may some day be utilized in 
driving dynamos to charge storage batteries, from which electricity 
can be taken when desired. 

The moon also helps the navigator to guide his ship, as explained 
in § 152. In this capacity the moon has frequently been likened to 
the hand of a stupendous clock, whose dial is the starry vault. 

In historical researches dates are frequently fixed by reference 
to eclipses, which inspired awe in the beholders, and were carefully 
recorded. The date of the beginning of the Christian era is deter- 
mined by means of a lunar eclipse, which took place on the night 
of Herod's death. The moon's light is of use in various ways, 
which readily suggest themselves. 



ECLIPSES. 

170. How Caused : Shape of the Shadow. — An eclipse of the moon 
occurs when it is in the shadow of the earth ; one of the sun is 
caused by the interposition of the moon between it and us. In 
order to understand them, we must study the shape of the shadow 
cast bv the earth or moon. 



THE MOON AND ECLIPSES. 



125 




In Fig. 115, S, E, and M represent the sun, earth, and moon, 

respectively. The heavily shaded portion CDV is called the umbra 

of the earth's shadow; it is of 

a conical shape. The lightly 

shaded portions, FCV and GDV, 

represent the penumbra of the 

shadow. An eye situated at X, 

between CF and CV, would see 

(neglecting the effect of refrac- 
tion of the sun's rays, where they 

graze the earth at C) only a 

portion of the sun's disk. An 

object between CF and CV would 

not be as brilliantly illuminated 

as one at the left of CF, where 

light from every part of the sun's 

disk would strike it. 

In Fig. 116, CHKD is the 

umbra of the moon's shadow; 

the penumbra occupies the space 

represented by FCH and KDG. 

A cross-section of the shadow 

of either the earth or the moon 

is shown in Fig. 117, the dark 

portion being the umbra, the 

lighter the penumbra. 

171. Cause of a Lunar Eclipse. — Since the centres of the sun and 
earth lie in the plane of the ecliptic, the axis 
of the earth's shadow, a line drawn from E 
to V in Fig. 115, lies there also. If the 
moon moved exactly in the plane of the 
ecliptic, it would pass through the earth's 
shadow every month, and suffer eclipse. But 
as the moon is above or below the ecliptic, 
except when at its nodes (§ 145) it usually 
passes above or below the earth's shadow 
and escapes eclipse. On those occasions 

when it encounters the shadow, the eclipse is total if the entire 




Fig. 115. — Umbra 
and Penumbra 
of the Earth's 
Shadow. 



Fig. 116. — Umbra 
and Penumbra 
of the Moon's 
Shadow. 




Fig. 117. — Cross-section 
of a Shadow. 




126 DESCRIPTIVE ASTRONOMY. 

moon passes into the umbra, and partial if only a portion of the 
moon is immersed in the umbra. 

172. Phenomena of a Total Lunar Eclipse. — When the moon is in 
the penumbra of the earth's shadow, enough sunlight still strikes it 
to make it shine brightly ; no one would surmise from its appear- 
ance that it was about to suffer eclipse. But as soon as it reaches 

the umbra, the portion of its limb in the dark 
shadow disappears from view, the moon having 
the appearance exhibited in Fig. 118. The 
dark notch grows until the entire moon is 
immersed in the umbra. But, strange to say, 
the whole moon usually becomes visible, shin- 
ing with a dull copper-colored light. The 
explanation is not far to seek. Many of the 
sun's rays pass through the earth's atmos- 

Fig. iiS.-Beginning of a here at C and D ( Fi ^-x an d, being re- 
Total Lunar Eclipse. r . ,,,,., , 

fracted, pass into the umbra and light up the 

moon with the sunset tinge. Should the earth's atmosphere be 
charged with clouds where the sun's rays attempt to struggle 
through it, the sunlight will be stopped by the clouds, and the 
moon will be entirely invisible ; this happens rarely. When the 
forward edge of the moon emerges from the umbra, totality is past; 
the eclipse ends when the entire moon has emerged from the umbra. 
Any phase of a lunar eclipse is visible from the whole of that hemi- 
sphere of the earth which is turned toward the moon. 

173. Cause of a Solar Eclipse. — A solar eclipse is caused, as 
shown in Fig. 116, by the moon's passing between the earth and 
the sun, so as to obscure the sun either partially or wholly. Since 
the moon does not move in the plane of the ecliptic, it does not 
get within the conical space ABCD (Fig. 115), every month, but 
usually passes above or below it. But when any part of the moon 
enters this conical space, the sun is at least partially obscured at 
some point of the earth's surface. 

174. The Moon's Shadow. — In Fig. 1 16, the moon's shadow, where 
it falls upon the earth, is quite narrow. On account of the variations 
of the earth's distance from the sun, and of the moon's distance from 
the earth, the moon's distance from the sun changes. The nearer it 
is to the sun, the shorter is its shadow (umbra) : the farther away, 



THE MOON AND ECLIPSES. 



> 

m 

— 

— 
rr 



^< 




THE MOON AND ECLIPSES. 



127 



the longer the shadow. Usually the shadow is not quite long enough 
to reach the earth's surface. Under the most favorable circum- 
stances, the diameter HK (Fig. 116) of the cross-section of the 
shadow at the earth's surface is 168 miles. The penumbra of the 
moon's shadow is shown by the light shading in Fig. 117. The 
moon hides a portion of the sun from an eye situated anywhere in 
the penumbra. The moon moves eastward, but as the earth turns 
in the same direction, the shadow does not skim over the continents 
as fast as it otherwise would. A projectile from a modern rifled 
cannon would keep up with it for a few seconds. 

175. Varieties of Solar Eclipses. — A total solar eclipse occurs 
when the whole sun is hidden from view. This happens only 
when the observer is within the umbra of the moon's shadow. 
Since the diameter of the cross-section of the umbra at the earth 




80- / S T 

Fig. 120. — Path of the Central Line of the Eclipse of May 27, 1900. 



is always less than 170 miles, the path of the shadow on the earth's 
surface is long and narrow; a total eclipse is visible only to those 
who are in this path. Fig. 116 shows that the penumbra is much 
wider; the average diameter of its cross-section at the earth is 
4,400 miles. 



128 



DESCRIPTIVE ASTRONOMY. 



For any one situated within the penumbra there will be a partial 
eclipse. The nearer he is to the true shadow path, the more of the 
sun will be hidden. The next total solar eclipse visible in the United 

States occurs on May 27, 1900. 

The path of the shadow is 

shown in Fig. 120. When the 

umbra is not long enough to 

reach the earth, any one at R 

in Fig. 121, where the axis of 

the umbra prolonged cuts the 

earth's Surface, Can look past Fig. 122. —Appearance 





the moon's edge and see a 



of the Sun during 
an Annular Eclipse. 



part of the sun, which will then 

have the appearance shown in Fig. 122. Such 

an eclipse is called annular. 

176. Phenomena of Partial and Annular Eclipses. 
— At the beginning of the eclipse, the moon 
appears to eat away the edge of the sun's disk, 
forming a notch similar to that shown in Fig. 
118 for a lunar eclipse. The notch increases 
to its maximum size, and then diminishes. One 
may get a good idea of the appearance by 
taking two equal circles, one black, the other 
white, and passing the black one slowly over 
the face of the white one, leaving a greater or 
less portion of the white one exposed to view. 
To represent an annular eclipse, the black circle 
must be smaller than the white one. 

With a telescope the lunar mountains are 
easily seen, projecting from the moon's limb 
where it eats into the sun. 

177. Phenomena of Total Eclipses. — A total 
eclipse is perhaps the grandest of natural phe- 
nomena. It begins in the same way as a partial one ; just before 
the sun is entirely covered, the landscape assumes an unearthly hue. 
Awe seizes the beholder: one sometimes sees the moon's shadow 
advancing through the air with terrifying swiftness, as if to smite 
him. In a few seconds it reaches him, and the last ray of sunlight 




Fig. 121. — Cause of an 
Annular Eclipse. 



THE MOON AND ECLIPSES. I 29 

is gone; the planets and bright stars appear. Around the black 
ball now hanging in the sky the pearly corona flashes out in all its 
weird beauty. At its base glow the prominences, like rubies set in 
pearl. Men's faces grow ghastly. The silence of death is upon the 
beholders. Soon there is a sudden flash of sunlight at the western 
limb of the moon: the corona and prominences fade apace. 

The gloom is overpast, and silence gives place to exclamations of 
wonder and delight. 

178. Observations During Totality. — Some of the more important 
of the observations made by astronomers are given below. 

1. Photographs of the corona and prominences are taken. 

2. The structure of the inner portions of the corona, which can be 
seen only during a total eclipse, is carefully studied with the telescope. 

3. Spectroscopic observations are made on the corona, the pro- 
tuberances, and the low-lying regions of the chromosphere. 

4. Search is prosecuted for possible small planets near the sun : 
it is claimed that such objects were seen during the eclipse of July 
29, 1878, by two American astronomers. 1 Diligent search has been 
made for these during more recent eclipses, but without success. 

5. Drawings are made of the outer corona to determine its extent 
and boundaries. 

179. Duration and dumber of Eclipses. — An eclipse of the moon, 
if total, may last for four hours. During half this time, the whole of 
its disk will be in eclipse. 

A total solar eclipse, from first to last contact, occupies about 
two hours. Totality may, on the rarest occasions, last nearly eight 
minutes. Its duration is ordinarily only two or three minutes. In 
some years there are no lunar eclipses : three may occur in a year, 
as will happen in 1898. 

Every year there are at least two solar eclipses ; there may be five. 

The greatest number of eclipses that can occur in any year is 
seven, of which two are lunar. 

Note. — The effects of a total solar eclipse on animals are interesting. Bees 
return to the hive. Chickens go to roost. Caged birds put their heads under 
their wings. Bats and owls fly out of their accustomed retreats. Dogs are terri- 
fied, and sometimes howl dismally. Horses have been known to lie down in the 
public highway and refuse to advance. Some oxen were once seen to range them- 
selves in a circle, back to back, with horns outward, as if to resist an attack. 

1 Lewis Swift and James C. Watson. 

9 



I30 DESCRIPTIVE ASTRONOMY. 



EXERCISES. 



180. 1. If the moon should cease to rotate on its axis, would its 
entire surface become visible to us? 

2. When the moon is new, does it rise and set at about the same 
time that the sun does? 

3. (a) When the moon is full, where is it to be looked for just 
after sunset? 

(b) Where just before sunrise? 

(c) Where at noon? 

(d) Where at midnight? 

4. (a) When the moon becomes a crescent, shortly after being 
new, does it set a little while before the sun, or after it? 

(b) Does it rise before the sun, or after? 

5. When the moon is at its first quarter, does the terminator (the 
straight edge) lie on the left hand side of the illuminated portion, or 
on the right hand side, as we look at it? 

6. When the moon is at its first quarter, does it cross the meridian 
a few hours before the sun, or a few hours after? 

7. When the moon is at its first quarter, and the sun is setting, 
do we look in the south for the moon, or in the east? 

8. Does the full moon shine all night, if the sky is clear? 

9. {a) W T hen the moon is in its last quarter, in what direction 
(north, east, south, or west) is it to be seen at sunrise? 

(b) In what direction at sunset? 

10. On the ecliptic are four cardinal points; viz. the vernal 
equinox, the summer solstice, the autumnal equinox, and the winter 
solstice. (§§ 98, 99.) 

(a) The sun being in the vernal equinox, if the moon were full, 
near what point of the ecliptic would it be ? 

(b) Where, if at first quarter? 

(c) Where, if at last quarter? 

11. If the moon on a given night be near either equinox, near 
what points of the horizon will it rise and set? 

12. The moon on a given night is near the summer solstice. 

(a) Will it, as seen from your home, rise in the northeast, or in 
the southeast? 



THE MOON AND ECLIPSES. I 3 1 

(b) When crossing the meridian, will it be near the zenith, or 
low down near the southern horizon? 

13. Is the moon visible an hour after sunrise, when it is at the 
last quarter? 

14. If the moon be full about Christmas time, will it run high 
(that is cross the meridian near the zenith), or low? 

[To answer this question first find, from the time of year, where 
the sun is in the ecliptic ; then from the relative positions of the sun 
and moon determine what point of the ecliptic the moon is near.] 

15. Why is not the dark part of the moon rendered plainly 
visible by earth shine at the first quarter, as well as when the moon 
is a narrow crescent? 

16. A mariner computes that upon a certain date the moon will 
occult the bright star Aldebaran, the disappearance occurring at 
8 h. 26 m. 47 sec, Greenwich mean time. By observing the occul- 
tation, he finds that his chronometer reads 8 h. 25 m. 58 sec. at the 
time of the star's disappearance. Is his chronometer fast or slow, 
and how mnch? 

17. Can a lunar crater ever be filled with the shadow of its own 
wall, so that the bottom of the crater will be invisible to us? 

18. Why are not the rays radiating from lunar craters overflows 
of lava? 

19. How does the atmosphere prevent our seeing stars by day 
as well as we see them at night? 

20. The lines AD and BV in Fig. 1 15 are tangent to the circle E. 
Are they tangent at exactly the same point? 

21. If the moon were as large as the earth, would it ever suffer 
a total eclipse? 

22. (a) At what kind (§175) of a solar eclipse does the moon 
look smaller than the sun? 

(#) At what kind does it look larger? 

23. If the earth were suddenly robbed of its atmosphere when 
the moon was visible during a total lunar eclipse, what change would 
there be in the moon's appearance? 

24. In a daily paper, of wide circulation, the head lines of an 
article on a solar eclipse read thus: "The Moon Casts its Shadow- 
on the Sun." Change that sentence so that it will express the 
truth. 



132 DESCRIPTIVE ASTRONOMY. 

25. When an observer at Chicago sees an annular eclipse, can 
an observer in some other city see the eclipse as total? 

26. (a) When a solar eclipse is partial for one observer, may it 
be total for another at the same time ? 

(J?) When a lunar eclipse is partial for one observer, may it be 
total as seen by another at the same time? 

27. The total solar eclipse of January, 1889, was visible in both 
California and Nevada. Did an observer on the coast of California 
see it before or after one in Nevada saw it? 

28. Does a total lunar eclipse end for an observer in Boston 
at the same instant as for one in Chicago ? 

29. What is the derivation of the word " annular " ? 

30. (ri) Do lunar eclipses happen when the moon is new? 
(J?) Do solar eclipses happen when the moon is new? 



MOTIONS OF THE PLANETS. 1 33 



CHAPTER VIII. 

MOTIONS OF THE PLANETS. 

" 'T is by the secret, strong attracting force, 
As with a chain indissoluble bound, 
The system rolls entire." 

Thomson. 

181. Their Orbits. — The orbit of each planet is an ellipse (§96), 
one focus of which is in the sun. The planes of all the planetary 
orbits, excepting those of some of the asteroids (§ 224), are but little 
inclined to the ecliptic. If a dot were placed in the centre of this 
page, to represent the sun, and all the planetary orbits were accu- 
rately drawn around it, the deviation of any one of them from true 
circularity would not be perceived. The definitions of major axis, 
minor axis, perihelion, aphelion, and mean distance, given in § 96, 
apply to the orbit of any planet. The radius vector of a planet is a 
line drawn from the focus of its orbit to the planet's centre. 

182. Motion in Orbit. — If one could station himself in space 
between the north star and the sun, and a billion miles from the 
latter, on looking back at the planets he would see that they were 
all moving about the sun in a direction opposite to that of the hands 
of a watch. This is an easterly motion. When a planet is at peri- 
helion, it moves more swiftly than at any other point of its orbit. 
The planet nearest the sun moves more rapidly than any other. 

183. Newton's Law of Gravitation. — This law, to which all bodies 
in the universe are supposed to be subject, may be stated in the 
following way. The mutual attraction between any two particles is 
proportional to the product of their masses, and inversely proportional 
to the square of t lie distance between them. 

This law may be expressed as an algebraic equation : let m and 
ml be the masses of two bodies, afthe distance between them, and k 
a number, the value of which depends on the units of mass and dis- 
tance employed. 



, 1)17)1 

Attraction = k — 
d 2 



134 DESCRIPTIVE ASTRONOMY. 

To make this clearer, suppose that two lead balls a mile apart 
attracted each other with a force of an ounce. If one ball were 
suddenly made five times as massive, the resulting attraction would 
be five times as great as before. If at the same time the other were 
made three times as massive, the new attraction between the bodies 
would not be 5 + 3, or 8 times the old attraction, but 5 X 3, or 15 
times the old attraction. Again, suppose that the masses of the 




Fig. 123. — Sir Isaac Newton. 

balls remained the same as at first, but the distance between the 
balls was doubled, the new attraction would not be ^ of the old, but 
the square of J, which is |, of the old. 

The force of gravity binds each planet to the sun. 

184. What keeps the Planets Moving ? — Persons ignorant of the 
laws of mechanics frequently think that gravity alone cannot keep 
the planets moving, but that some other force is pushing them. 
But there is no such extra pushing force. One of the laws of 
mechanics is that a body once set in motion will continue to move 



MOTIONS OF THE PLANETS. 



135 



in a straight line with a uniform velocity, unless acted on by some 
external force. The tendency to keep going if once set in motion, 
or to remain at rest if stopped, is known as inertia. So a planet 
needs no pushing force behind it : having in some way been set in 
motion, its tendency is to move in a straight line ; but the gravita- 
tional pull of the sun compels it to describe a curve instead. 




Fig. 124. — Kepler. 



185. Kepler's Laws. — Before the time of Kepler, who was born 
in 1 571, the heavenly bodies were supposed to move in circles. He 
discovered three laws concerning the motions of the planets: — 

I. The orbit of each planet is an ellipse, the sun being at one of 
its foci. 

II. The radius vector of a planet describes equal areas in equal 
times. 



136 



DESCRIPTIVE ASTRONOMY. 



III. The squares of the times of revolution of any two planets 
are to each other as the cubes of their mean distances from the sun. 

The second law is illustrated in Fig. 
125. If a planet describes the arcs AB 
and CD in the same length of time, the 
area of SAB is equal to that of SDC. 

Let t and f denote the times of revo- 
lution of two planets, while a and a' are 
their mean distances. Then, by the third 




Fig. 125. — Equal Areas in 
Equal Times. 



The discovery of this law, known as 
the harmonic law, after seventeen years of 
arduous labor, caused Kepler the great- 
est exultation. He wrote concerning it : 
" The die is cast : the book is written, 
to be read either now or by posterity, — 
I care not which. It may well wait a 
century for a reader, as God has waited six thousand years for an 
observer." 

186. Perturbations. — Gravitation being universal, it follows that 
the planets attract each other. These attractions cause disturbances 
of their elliptic motions. The computation of these perturbations 
has taxed the highest skill of mathematical astronomers ; by a series 
of profound and elegant researches it has been proved that the 
stability of the planetary system is not endangered by them. 



APPARENT MOTIONS. 

187. Two Classes of Planets. — For convenience in discussing their 
apparent motions the planets are divided into two classes. The 
inferior planets are those the orbits of which lie within that of the 
earth : these are Mercury and Venus. The superior planets are 
those whose orbits are exterior to that of the earth : they are Mars, 
the asteroids, Jupiter, Saturn, Uranus, and Neptune. 

188. Aspects. — One is aided in remembering the following ex- 
planations of the aspects of the planets by the thought that they all 
refer to the position of a planet with relation to the sun, as we, 
looking out from the earth, see the two bodies. 



MOTIONS OF THE PLANETS. 



137 



Conjunction 1 occurs when a planet appears to be close to the 
sun. A superior planet is then beyond the sun. An inferior planet 
may be beyond the sun, in which case it is in superior conjunction, 
or it may be between the earth and the sun ; in that case it is in 
inferior conjunction. Conjunction is indicated by the sign 6 • 

Opposition occurs when the planet is in the opposite direction 
(from us) to that in which the sun lies. If the sun were near the 




opposition 

Fig. 126. — Aspects of the Planets. 



east point of the horizon, a planet in opposition would be near the 
west point. Opposition is denoted by the sign <?. 

A planet's elongation from the sun is the angle formed at the 
earth by two lines drawn from it to the planet and the sun respect- 

1 There are different kinds of conjunction, opposition, etc., because the planets* 
orbits do not coincide with the ecliptic. Planets are at conjunction in longitude when 
their longitudes are the same: they are in conjunction in right ascension when their 
right ascensions are the same. 



138 DESCRIPTIVE ASTRONOMY. 

ively. The greatest elongation of an inferior planet is illustrated in 
Fig. 126. 

A superior planet is in quadrature when its elongation is 90 . 
Quadrature is denoted by the sign D. 

189. Apparent Movement of an Inferior Planet. — In Fig. 126 the 
orbit of Venus is drawn to illustrate that of an inferior planet. After 
being at inferior conjunction, Venus, moving in a direction opposite 
to the hands of a watch, goes to its greatest western elongation. 
If the earth stood still, Venus would arrive at its elongation in four 
weeks ; but as the earth chases after it, the interval between inferior 
conjunction and greatest western elongation is lengthened to 2\ 
months. The planet then passes on through superior conjunction 
and greatest eastern elongation to inferior conjunction again. 

While it is travelling from inferior to superior conjunction, a man 
facing the sun will see the planet at the right (or west) of the sun. 
During the other half of its course it will be east of the sun. 

190. Apparent Movement of a Superior Planet. — In Fig. 127 are 
represented the orbits of the earth and of Mars, P, P', and P" being 
on the celestial sphere. E and M are the positions of the earth and 
Mars when the latter is at opposition. Mars then appears to be at 
the point P on the celestial sphere. Two weeks thereafter the 
earth has moved to E', and Mars, moving more slowly on account of 
its greater distance from the sun, has traversed the arc M M'. Mars 
then appears to be at P', which is west of P. So a superior planet, 
though really moving eastward in its orbit, appears when near oppo- 
sition to move toward the west among the stars, because its motion 
is slower than that of the earth, and the two bodies are moving in 
nearly the same direction. This westward motion is said to be the 
retrograde. 

Again, let Mars at M be in conjunction, the earth being at E", 
and Mars appearing to us to be at P. In a few days the earth ar- 
rives at E //r and Mars at M', so that it appears to be at P", having 
moved east from P. Had the earth remained at E", Mars in moving 
from M to M' would have appeared to go east, but would not have 
reached P". 

Summing the matter up, we reach three conclusions: — 
I. If the earth were stationary, the planet's easterly motion in its 
orbit would cause it to appear to move eastward among the stars. 



MOTIONS OF THE PLANETS. 



39 



II. When the planet is near opposition, the more rapid motion 
of the earth causes its eastward motion to be apparently reversed, 
so that it retrogrades or moves westward among the stars. 

III. When the planet is near conjunction, its apparent eastward 
motion among the stars is swifter than it would be, were the earth at 
rest. 




Fig. 127. —Apparent Movement of a Superior Planet. 



When changing its apparent eastward motion to a westward, and 
vice versa, the planet is said to be at a stationary point. 

191. Evening and Morning Stars. — When a planet rises between 
midnight and the ensuing sunrise, it is called a morning star. When 



I40 DESCRIPTIVE ASTRONOMY. 

it is above the horizon at some instant between sunset and the fol- 
lowing midnight, it is an evening star. Hence an evening star can 
be seen before midnight (if not too near the sun), and a morning 
star cannot. 

192. Periods, Sidereal and Synodic. — The sidereal period of a 
planet is the time of a complete revolution around the sun. The 
synodic period is the time which elapses between two successive 
conjunctions of the planet with the sun. If the planet be an inferior 
one, the two conjunctions must be both inferior or both superior. 

EXERCISES. 

193. 1. (a) At what point of its orbit is a planet, when its radius 
vector has its greatest length? 

(b) At what point when it is the shortest? 

2. (a) Why does a planet move most swiftly when at its peri- 
helion ? 

(b) Why does Mercury travel more miles in an hour than any 
other planet? 

3. Two balls, a rod apart, attract each other with a force of one 
grain. If the mass of one ball be made ten times as great, while that 
of the other is halved, what will be the attraction between them, the 
distance remaining the same? 

4. In exercise 3 what would have been the mutual attraction had 
the original balls been placed ten rods apart? 

5. What would have been the mutual attraction had the original 
balls been placed one fourth of a rod apart? 

6. What would have been the attraction, if each ball had been 
halved, and the distance had been halved also? 

7. What would have been the attraction if the mass of one ball 
had been made three times as great, while that of the other was 
made ten times as great, and the distance between them shortened 
to one fifth of a rod ? 

8. If the gravitational pull between the earth and the sun were 
suddenly to cease, how would the former move? 

9. How does Kepler's second law show that a planet when at 
aphelion must describe a shorter arc of its orbit in a day than when 
at perihelion ? 



MOTIONS OF THE PLANETS. 1 4 1 

io. The mean distance of the earth from the sun being 93 millions 
of miles, while that of Jupiter is 483 millions, show by Kepler's third 
law that the period of Jupiter's revolution about the sun is nearly 
twelve years. 

11. The mean distance of Neptune being thirty times that of the 
earth, show that its period is over 164 years. 

12. The period of Uranus being eighty-four years, show that its 
mean distance from the sun is over nineteen times that of the earth. 

13. Does a superior planet which is in conjunction set about 
sunrise? 

14. Does a planet when in opposition rise about sunset? 

15. (a) At about what time of day does an inferior planet, when 
in inferior conjunction, cross the meridian? 

(b) At what time, when in superior conjunction? 

16. What is the aspect of a superior planet which is on the 
meridian at midnight? 

17. Why does a superior planet look brightest when at oppo- 
sition? 

18. Why are planets not easily observed when they are in con- 
junction? 

19. When Mercury is at its greatest eastern elongation, being 
28 from the sun, is it visible in the evening twilight? 

20. If Mercury was seen going across the face of the sun, would 
it move from the sun's eastern limb towards its western, or vice 
versa ? 

21. If Venus is at its greatest eastern elongation, being 47 from 
the sun, does it rise before the sun? 

22. When at its greatest western elongation, is Venus a morning 
star or an evening star, or both? 

23. Just after an inferior planet passes its superior conjunction, is 
it a morning star or is it an evening star? 

25. If Venus is at its eastern elongation, does it cross the upper 
branch of the meridian in the forenoon, or in the afternoon? 

26. Could Jupiter and Venus ever appear to be close together? 

27. At one of its oppositions Mars was near perihelion, while 
the earth was near aphelion. At another opposition Mars was 
near aphelion, while the earth was near perihelion. At which oi 
the two oppositions could Mars be best seen by us? 



I42 DESCRIPTIVE ASTRONOMY. 

28. When does a superior planet appear to have the smaller 
diameter, at opposition or at conjunction? 

29. When does an inferior planet appear to have the smaller 
diameter, at inferior conjunction or at elongation? 

30. Should an inferior planet, shining by reflecting the sun's 
light, show phases similar to those of the moon? 

31. Draw a picture containing two concentric circles, one repre- 
senting the orbit of the earth, the other that of a superior planet. 
Mark the positions of the sun, earth, and planet, when the planet 
is in conjunction with the sun. Determine whether the synodic 
period of the planet is longer than the sidereal. 

32. Find, by making a drawing similar to that described in the 
preceding exercise, whether the sidereal period of an inferior planet 
is longer than the synodic. 



MERCURY, VENUS, MARS, THE ASTEROIDS. 



143 



CHAPTER IX. 

MERCURY, VENUS, MARS, THE ASTEROIDS. 

" Now glowed the firmament 
With living sapphires : Hesperus, that led 
The starry host, rode brightest." 

Milton. 

194. Two Groups of Planets. — When the planets themselves are 
considered, instead of their orbits, they fall naturally into two divis- 
ions. Mercury, Venus, the earth, and Mars are all comparatively 
small bodies, are doubtless solid, and are quite dense. Each is sup- 



S^iufn 




Uranus 



The E&rrk • M&rs 
Venus • Mercury 



Fig. 12J 



Relative Sizes of the Planets. 



posed to have an atmosphere of small mass compared with the mass 
of the body enveloped by it. Their equipment of moons is meagre. 
The relative sizes of the planets are shown in Figure 128. 

Jupiter, Saturn, Uranus, and Neptune, in comparison with the 
other planets, are giants in size. But their densities are small, so 



144 DESCRIPTIVE ASTRONOMY. 

that they have perhaps only little kernels of solid matter at their 
centres. Their atmospheres are very extensive and dense. They 
are liberally provided with satellites, except Xeptune. Were he 
nearer, we might discover that he had a goodly retinue of them. 
The planets just mentioned, eight in all, are called the major planets. 
The minor planets, or asteroids, are quite insignificant in point of 
size. Very little is known of their physical constitution. 



MERCURY, £. 

195. Distance and Diameter. 1 — The mean distance of Mercury 
from the sun is 36,000000 miles. The actual distance varies 
7,500000 miles each side of this value, the orbit being much more 
eccentric than that of any other of the large planets. 

The diameter of Mercury is 3,000 miles. 

196. Revolution and Rotation. — The sidereal period is 88 days, 
so that it performs a revolution in less than one fourth of the time 
required by the earth. 

The time of its rotation upon its axis cannot be said to be cer- 
tainly known. One astronomer, a century ago, thought he saw 
certain appearances on the planet due to the presence of high 
mountains; by observing them, he obtained a rotation period of 
about 24 hours. But his observations have not been confirmed by 
more powerful telescopes. 

Schiaparelli, a distinguished Italian astronomer, who has made a 
special study of some very faint markings on Mercury since 1881, 
has concluded that Mercury rotates on its axis in the same time 
that it revolves about the sun. Thus, as our moon continually pre- 
sents the same face to the earth, Mercury turns the same side to the 
sun. Schiaparelli announced his discovery to a friend in 18S2 in the 
following lines : — 

" Cynthiae ad exemplum versus Cyllenius axe 
Aeternum noctem sustinet, atque diem : 
Altera perpetuo facies comburitur aestu. 
Abdita pars tenebris altera Sole caret.'' 

1 The distance, diameter, sidereal period, and rotation time of each planet, should be 
thoroughly committed to memory. 



MERCURY, VENUS. 145 

197. Transits. — When Mercury is near inferior conjunction, it 
sometimes gets into line between the earth and the sun, so that it is 
seen by us as a small black circle crossing the solar disk. Thirteen 
transits occurred during the nineteenth century, the last one being 
on Nov. 10, 1894. 

198. Appearance to the Naked Eye. — The planet keeps so close 
to the sun, that it is not readily seen with the naked eye. The times 
of its elongation are the best times to look for it: it can be well 
seen about a week before elongation, as well as a week after. The 
dates of elongation are given in the Nautical Almanac. The best 
conditions for seeing it in the evening occur at those eastern elonga- 
tions which happen in March or April. It then appears like a star 
of exceptional brilliancy, near the western horizon, distinguishable 
in strong twilight, and conspicuous as soon as night sets in. 
Copernicus is said never to have seen it. 

199. Telescopic Appearance. — The telescope shows that the planet 
has phases like the moon : it therefore shines by reflecting the light 
of the sun. When near inferior conjunction it is a narrow cres- 
cent, as is the moon when new. Its phase at superior conjunction 
is like that of the full moon. Favorable views of the planet are 
rare, since it must be observed either during the daytime, or at 
night when it is near the horizon. Faint dark markings are some- 
times seen on its disk, but they are so indistinct that their nature 
can only be guessed at. They may be dark plains, like those on the 
moon, or possible lakes or seas. 

200. Physical Condition. — There is spectroscopic evidence of the 
presence of water vapor; from this we conclude that both air and 
water are to be found on the planet. But it is probable that the 
atmosphere is not as dense as ours. The sun shines seven times 
as hotly as on the earth. 

VENUS, 9. 

201. Morning and Evening Star. — Venus is the most brilliant of 
the planets, and when at its maximum brightness casts distinct 
shadows of objects at night, in the absence of the moon or bright 
artificial lights near at hand. It is then visible to the naked eye in 
full daylight. Because of its brightness it has received the distinct- 
ive appellations of the Evening Star and the Morning Star. The 



146 DESCRIPTIVE ASTRONOMY. 

Greeks called it Hesperus when it was an evening star, and Phos- 
phorus when it was a morning star. Of late years many ignorant 
people, seeing it by day, have supposed it to be a reappearance of 
the Star of Bethlehem. 

202. Distance and Diameter. — The mean distance of Venus from 
the sun is 67,000000 miles ; its distance, when in different parts of 
its orbit, varies little, because its orbit is more nearly circular than 
that of any other planet. The planet's diameter is 7,700 miles : it 
is therefore nearly as large as the earth. When at inferior conjunc- 
tion it is nearer to us than any other planet ever is. 

203. Revolution and Rotation. — Venus accomplishes a revolution 
about the sun in 225 days. The time of its rotation is not now in 
much doubt. Until lately the only evidence (and that very insuffi- 
cient) was that it revolved in 23 h. 21 m. But the recent researches 
of Schiaparelli, the keenness of whose vision is remarkable, render 
it probable that the time of rotation is 225 days, agreeing with the 
sidereal period. This result has been corroborated by two other 
Italian astronomers. 

204. Transits. — Transits of Venus across the sun's face are much 
rarer than those of Mercury. The last one occurred on Dec. 6th, 
1882, and the next one will not come until 2004. Another is due 
in 2012. These transits have a high degree of interest, because they 
have been used in finding the distance of the earth from the sun. 
Expensive scientific expeditions have been sent to various parts of 
the world, by the governments of the most progressive nations, to 
observe these transits. 

Halley's l method of observation consists of observing the times 
of external and internal contact as seen from two stations widely 
different in latitude. As shown in Fig. 129, the planet as seen from 
these stations has different paths across the sun's disk. The dis- 
tance and direction of one station from the other being known, and 
the lengths of the two paths being measured, it is possible by trig- 
onometric methods, too difficult to be explained here, to find the 
sun's distance. The accuracy of the final result depends upon the 
precision with which the times of contact are noted. 

Unfortunately, when the black circle of the planet's disk is inter- 
nally tangent to the sun's limb at the beginning of the transit, it has 

1 Halley was an English Astronomer Royal in Newton's time. 



VENUS. 



H7 



the appearance shown in Fig. 130, As it moves away from the 
limb, it is, for a number of seconds, apparently attached to it by a 
black ligament, called the " black drop." The ligament stretches, 
contracts, and finally breaks. Thus it is very difficult to note the 




VENU5 




BLACK 



DROP 



Fig. 129. — A Transit of Venus. 



Fig. 130. — The Black Drop. 



time of real internal contact. The action of the planet's atmos- 
phere also vitiates the accuracy of the observation. A phenomenon 
similar to the black drop may be seen by placing the thumb and 
forefinger close together, and holding them six inches or less from 
the eye. 

On account of these troubles astronomers now place more re- 
liance upon other methods. 

205. Phases and Maximum Brightness. — The phases of Venus 
are similar to those of Mercury and the moon. They are almost 
visible to the naked eye. A good spyglass brings out the crescent 
phase well. The time of greatest brightness does not occur when 
the planet looks like a full moon, for then it is farthest from us. It 
comes during the crescent phase, five weeks before, and the same 
time after, inferior conjunction. 

The discovery of the phases of Venus was one of the first fruits 
of the invention of the telescope. Galileo 1 made the discovery and 
announced it in the following anagram : " Haec immatura a me jam 
frustra leguntur, o. y." This he afterwards transposed so that it 
read, " Cynthiae figuras aemulat mater amorum." 



1 Galileo Galilei (1564- 1642), the famous Italian philosopher. 



148 



DESCRIPTIVE ASTRONOMY. 



206. Telescopic Appearance. — The planet is of dazzling splendor, 
even in a small telescope ; it looks almost as if made of quicksilver, 




Fig. 131. — Galileo. 



and is surrounded by a marked purplish aureole caused by the 
lack of achromatism (§ 39) of the telescope. 

On rare occasions ill defined spots of a leaden hue are seen on 
its surface. They may be continents or seas dimly descried. Cer- 
tain very bright spots, said by some to be occasionally visible near 
the limb, have been thought to be due possibly to polar ice and 
snow. 

207. Atmosphere. — When Venus is near inferior conjunction, 
being a very slender crescent, the horns or cusps of the crescent 
appear to be much prolonged, so that they really surround the 
dark disk of the planet. When about to enter upon a transit, a ring 
of light is seen surrounding the entire disk (Fig. 132). This is 



VENUS, MARS. I 49 

the sunlight shining through the planet's atmosphere, and being 
refracted by it to our eyes. The atmosphere has been shown to be 



Fig. 132. — The Ring of Light. 

denser than ours, and probably less than twice as dense. The vapor 
of water has been detected in it by spectroscopic observations. 

208. Physical Condition. — The density of Venus being nearly 
equal to that of the earth, we conclude that it is a solid body. It 
probably owes its brilliancy to the fact that its sky is almost totally 
cloudy at all times. Any one looking at bright white masses of 
cumulus cloud in a summer sky will be convinced that such clouds 
reflect light much better than the general landscape does. The 
excessive cloudiness in turn, combined with the spectroscopic evi- 
dence of water vapor, indicates that water is abundant on the 
planet's face. There may not be a square foot of dry land to 
vary the monotony of a universal ocean. 

MARS, $. 

209. Distance and Diameter. — The mean distance of Mars from 
the sun is 141,500000 miles. Its orbit is, excepting Mercury's, the 
most eccentric of all the orbits of the major planets, so that the 
planet's distance varies 13,000000 miles each side of the average 
distance. Its diameter is 4,200 miles, which is not much more than 
half the earth's diameter. 

210. Revolution and Rotation. — The sidereal period is 6S7 days, 
only 43 days short of two years. By comparison of drawings of the 



I50 DESCRIPTIVE ASTRONOMY. 

planet made soon after the invention of the telescope with those 
made during this century, the rotation time has been determined 
with great precision. It is 24 h. 37 m. 22.67 sec - 

211. Appearance to the Naked Eye. — Mars, being a superior 
planet, is best seen at the time of opposition, when it is near the 
earth. At some oppositions it comes within 36,000000 miles of us, 
at others it is as much as 61,000000 miles away. This variation of 
distance is due to the eccentricity of the orbit. The favorable oppo- 
sitions, which come when the planet is near its perihelion, occur 
about every fifteen years. The last was in August, 1892. At such 
times Mars is a brilliant object, shining with a fiery red light, and 
fairly rivalling Jupiter in splendor. It is then more than fifty times 
as bright as when faintest, at conjunction. When not near opposi- 
tion, it might frequently be mistaken by an unpractised eye for one 
of the brightest of the fixed stars. Its motion among them would 
lead to its speedy identification. No other planet looks red, except 
when near the horizon. 

212. Phases : Appearance in a Small Telescope. — When at oppo- 
sition the planet's disk looks round, as seen with a small telescope, 
but at quadrature it is plainly gibbous. For at that time we are not 
in line between Mars and the sun, and so do not see all of its 
illuminated hemisphere. ( See exercise 2 1 at the end of this chapter.) 
Besides the phase, an eye armed with a small telescope (three or 
four inches in aperture) sees at opposition that the surface is not all 
red, but bears certain darker markings, generally thought to be of 
an olive-green hue. At times a small white spot may be seen at 
one of the poles. The dark spots are supposed to be due to the 
presence of water ; the white polar spot suggests snow. 

213. The Polar Caps. — The caps are generally believed to be 
composed of snow and ice, not only because of their white appear- 
ance and their situation, but also because the northern one dimin- 
ishes in size during summer time in Mars's northern hemisphere, and 
increases during the winter. The southern cap behaves in a similar 
fashion. During the opposition of 1892, the southern polar cap 
seemed to diminish with great rapidity. Its area was estimated to 
lose 1,500000 square miles in a month. First a dark spot was seen 
in the snow: this spot gradually enlarged, splitting the cap into two 
parts, each of which melted away at an astonishing rate. On several 



MARS. 



151 



occasions white spots, apparently detached snowfields, were seen 
lying close to the main cap. 

In 1894 a similar melting took place: in May hundreds of square 
miles of the polar cap disappeared daily. During the melting a 
dark band surrounded the cap, keeping at its edge continually, as 
would be expected if the snow and ice were turning into water. In 
October the cap had become so small that it was seen with the 




Fig- l 33- — Mars: Drawn by Barnard. 

Lick telescope only ; the remnant of it seemed to be almost hidden 
by some overhanging veil. 

Fig. 133 shows the planet as it appeared to Dr. E. E. Barnard 1 
with the Lick 36-inch telescope on August 19, 1892. The polar 
cap was then only one third as large in area as in June of the same 
year. 

1 Now of the Yerkes Observatory. 



152 DESCRIPTIVE ASTRONOMY. 

214. Seas. — The dark areas, if really seas, as generally supposed, 
are not as permanent in form as the oceans on the earth. The 
permanent water area has been estimated at about 500,000 square 
miles, which is only half as great as that of the Mediterranean Sea. 
When the polar cap melted in the summer of 1892, a portion of 
the region between it and one of the seas became dark, and the sea 
apparently increased in size. While we cannot be confident about 
the cause of such changes, it has been suggested that the water 
produced by the quick melting of the polar cap flowed across the 
land to a sea, increasing its size temporarily. 

Though some of the dark portions of the planet's surface are 
probably bodies of water, there is reason to believe that much of 
the dusky area is not permanently covered with water. For 
canals (to be described later) have been seen in these " seas," 
and many different shades of color exist there : the same regions 
have different tints at different times. Sometimes a vast amount 
of detail is perceived, which would scarcely be found upon a water 
surface. 1 

215. Continents and Islands. — The reddish portion of the planet's 
disk is supposed to be dry land. But these hypothetical continents 
are not secure in their boundary lines. Disappearances of portions 
of them have been noted ; they seem to be inundated by the waters 
of neighboring seas. Dr. S. P. Langley, in his work entitled " The 
New Astronomy," states that Lockyer Land is sometimes seen 
white, as if covered with ice ; further, that Hall Island has this white 
appearance so frequently as to suggest the idea that some mountain 
or table-land on it rises into the region of perpetual snow. The 
changes on the surface of Mars during the opposition of 1892 were 
so noteworthy that Dr. E. E. Barnard was led to write as follows : 

1 Dr. Barnard describes some of his observations with the Lick telescope in 1894 
as follows : — 

" Under the best conditions these dark regions, which are always shown with 
smaller telescopes of nearly uniform shade, broke up into a vast amount of very fine 
details. I hardly know how to describe the appearance of these ' seas ' under these 
conditions. To those, however, who have looked down upon a mountainous country 
from a considerable elevation, perhaps some conception of the appearance presented 
by these dark regions may be had. From what I know of the appearance of the coun- 
try about Mount Hamilton, as seen from the observatory, I can imagine that, as viewed 
from a very great elevation, this region, broken by canyon and slope and ridge, would 
look just like the surface of these Martian 'seas.' " 



MARS. 153 

" These striking changes are enough to make us pause and question 
whether what we see before us in the heavens is really another 
world like our own, with relatively fixed oceans and continents, 
or whether it is not a world like our own in its younger days, 
when continents were shifting and oceans changing, before the 
surface of the earth became firm and fixed by the process of 
cooling." 

Some of the apparent changes in the forms of the continents 
have been ascribed to the spread of vegetation along their borders. 

216. Clouds. — Transient spots, having the general aspect of 
cloud masses, have been observed. Sometimes they are small and 
of tolerably definite outline, but usually they are diffuse and of 
large extent. They have also appeared as long streaks projecting a 
trifle beyond the planet's limb. At times portions of the land- 
scape have been so obscured as to give rise to the theory that the 
obscuration was caused by a passing cloud. But many details of 
the Martian landscape are usually seen so plainly that clouds must be 
considered as rarities. Their comparative absence would naturally 
follow from the small proportion of water surface. 

217. Atmosphere. — Were the atmosphere dense, like that of 
Venus, we should never have discovered the mass of topographical 
detail now known. The rarity of Mars's atmosphere has been ac- 
counted for by the moderate size of the planet, and the weakness of 
the force of gravity at its surface. Could a rifle ball be shot up- 
ward with a velocity of 3.5 miles a second, it would, unless checked 
by atmospheric resistance, leave the planet never to return. The 
speed of molecules of hydrogen, in their incessant vibration, may 
considerably exceed this, so that free hydrogen is not to be looked 
for as a constituent of Mars's atmosphere. The best spectroscopic 
observations indicate that the atmosphere of Mars exerts no meas- 
urable absorptive effect upon the sunlight which strikes through it, 
and is reflected back to us. We are therefore ignorant of its com- 
position, and can only say that, if it be similar to that of our air, its 
average density can scarcely be one fourth as great. 

218. Description of the Canals. — In 1877 Schiaparelli discovered 
several of the markings which are commonly called " Schiaparelli's 
canals." A few had been seen previously. Many more have since 
been found by him and by others. The map of Mars made at the 



*54 



DESCRIPTIVE ASTRONOMY. 



Lowell observatory 1 in 1894, exhibits a bewildering network of 
canals, connecting small dark spots scattered over the surface. Not 
infrequently half a dozen canals radiate from a single spot, going 
straight to other spots. Most of the canals choose the shortest 
path from one spot to another : a few are curved. Some do not 
run from one small dark spot to another, but connect large dark 
areas, or go from a small spot to a large area, or occasionally con- 
nect two other canals. The small spots are less than 150 miles in 
diameter. The length of the canals ranges from a few hundred to 
3,500 miles. Their average breadth is 30 miles. The most myste- 
rious fact about them is that they become double at times, the 
two new canals being about 200 miles apart, and veritable twins. 
Schiaparelli thinks that the doubling may be periodical, and con- 
nected in some way with the planet's seasons. 

219. Explanations of the Canals. — The canals have naturally been 
supposed to be water-ways. When a polar cap melts, the canals 
in the neighborhood become darker and wider, and remain dark 
until the snow stops melting. Then the width of the canals 
diminishes. These appearances have led Schiaparelli to the con- 
clusion that the canals are natural furrows, through which the 
water is carried from the poles equatorward. Mr. Percival Lowell 
advocates the theory that the canals are strips of vegetation, which 
are watered by canals too small to be visible to us. A small spot 
at the junction of several canals is an oasis, according to this view. 
No satisfactory explanation of the doubling of the canals has 
been given. The majority of astronomers, while freely admitting 
the existence of the markings called canals, are inclined to be 
conservative with reference to any explanation of their nature. It 
has been aptly said that it is better not to know so much, than to 
know so many things that are not so. 

220. Colors. — Orange and grayish green are the prevailing col- 
ors, outside of the polar caps. But various colors have been seen 
in different spots. The same spot has been of different hues at 
different times, though the utmost care was taken to avoid optical 
illusions. Light greens and bright greens have been seen often. 
At times places supposed to be bodies of water have exchanged 

1 A temporary observatory set up at Flagstaff, Arizona, by Mr. Percival Lowell, 
of Boston. 



MARS. 



155 




156 DESCRIPTIVE ASTRONOMY. 

their ordinary color for dark blue. Gray and yellow tints are of 
common occurrence. Even so extraordinary a color as violet-lake 
was once perceived. Some of these colors may be explained as 
due to haziness or partial cloudiness. Perhaps some are due to 
the presence of vegetation. 

221. Satellites. — Mars is attended by two moons, discovered by 
Prof. Hall x in August, 1877. Their names are Deimos and Phobos. 2 
The distance of Deimos from the planet's centre is 14,600 miles; 
it completes a revolution about Mars in 30 h. 18 m. 

Phobos is at a distance of but 5,800 miles and takes only 7 h. 
39 m. for one revolution. The time of rotation of Mars being 
over 24 hours, Phobos by reason of its rapid motion rises in the 
west and sets in the east, as our moon would if its orbital motion 
were swift enough. 

These moons are so minute that it is not possible to measure 
their diameters directly ; but by measures of the amount of light 
that they give, Prof. Pickering 3 has concluded that the inner one 
is 7 miles in diameter, the outer one 5 or 6. Estimates made at the 
Lowell observatory give a diameter of 36 miles for Phobos and 10 
miles for Deimos. Their discovery was as great a feat of telescopic 
vision as for a man in Boston to see a tennis ball at Philadelphia. 

222. Habitability. — If we have simply to answer the question, 
" Would a man, as constituted at present, if transported to Mars, 
find it possible to exist there?" the most probable answer is, 
" No." While one must not be dogmatic, it may be said, with 
some assurance, that the man would gasp a few times and die. 
However, it is conceivable that manlike beings might find a home 
there. Plans for communication with the supposititious inhab- 
itants of Mars by means of huge signals displayed in deserts or on 
table-lands, or by gigantic combinations of electric lights, are little 
better than phantasies of a disordered imagination. If some enter- 

1 Prof. Asaph Hall, formerly of the U. S. Naval Observatory, Washington, D. C. 

2 These names were given by Homer to the steeds which drew the chariot of the 
god of war. In one passage in the fifteenth book of the Iliad they are personified, 
and refer to the attendants of Mars. Bryant's translation is : — 

" He spake, and summoned Fear and Flight to yoke 
His steeds and put his glorious armor on." 

3 Edward C. Pickering, Director of the Harvard College Observatory. 



MARS. I57 

prising and athletic individual on Mars should wave a flag as large 
as the State of New York, terrestrial astronomers might notice 
his greeting. 

There have been curious anticipations of the discovery of the 
moons of Mars. 1 

Kepler, in a letter written after the discovery of four satellites of 
Jupiter by Galileo, said : " I am so far from disbelieving the 
existence of the four circumjovial planets, that I long for a tele- 
scope to anticipate you, if possible, in discovering two around 
Mars, as the proportion seems to require, six or eight around 
Saturn, and perhaps one each around Mercury and Venus." 

Swift, in his satire, "The Travels of Mr. Lemuel Gulliver," puts 
the following language into the mouth of Gulliver, who had arrived 
among the Lilliputians : — 

"The knowledge I had in mathematics gave me great assistance in 
acquiring their phraseology, which depended much upon that science, and 
music : and in the latter I was not unskilled. Their ideas were perpetu- 
ally conversant in lines and figures. If they would, for example, praise 
the beauty of a woman, or any other animal, they describe it by rhombs, 
circles, parallelograms, ellipses, and other geometrical terms, or by words 
of art drawn from music, needless here to repeat. . . . They spend the 
greatest part of their lives in observing the celestial bodies, which they do 
by the assistance of glasses far excelling ours in goodness. For although 
their largest telescopes do not exceed three feet, they magnify much more 
than those of a hundred with us, and show the stars with greater clearness. 
This advantage has enabled them to extend their discoveries much farther 
than our astronomers in Europe ; for they have made a catalogue of ten 
thousand fixed stars, whereas the largest of ours do not contain above one 
third of that number. They have likewise discovered two lesser stars or 
satellites which revolve about Mars : whereof the innermost is distant from 
the centre of the primary planet exactly three of his diameters, and the 
outermost five ; the former revolves in the space of ten hours, and the 
latter in twenty-one and a half; so that the squares of the periodical times 
are very nearly in the same proportion with the cubes of their distances 
from the centre of Mars ; which evidently shows them to be governed by 
the same law of gravitation that influences the other heavenly bodies." 

1 The following interesting information is derived from Professor Hall's monograph 
on the satellites. 



158 DESCRIPTIVE ASTRONOMY. 

Voltaire represents, in one of his works, that Micromegas, an 
inhabitant of Sirius who visited our system, discovered that Mars 
had two moons, which made perpetual compensation for the lack of 
the brilliant sunlight which we enjoy. 

THE ASTEROIDS, OR MINOR PLANETS. 

223. Bode's Law. — In 1772 the astronomer Bode brought into 
prominence a relation betw r een the distances from the sun of the 
then known planets. This relation had been discovered some years 
previously by Titius, but it is commonly called Bode's Law, and is 
found as follows. The series o, 3, 6, 12, etc., in which each number 
except the first is double the preceding one, is written. To each 
term 4 is added. 

03 6 12 24 48 96 192 

4 7 10 16 28 52 100 196 

The resulting numbers represent fairly the relative distances. 
Taking the earth's distance as ten, the real distances are given 
below. 



Mercury . . 


• 3-9 


Jupiter . . 


• 5 2 -° 


Venus . . 


. 7.2 


Saturn . . 


• 95-4 


Earth . . . 


10. 


Uranus . . 


. 191.8 


Mars . . . 


• i5- 2 







The number 28 in the scheme of Titius corresponded to no known 
planet. Astronomers generally became imbued with the notion that 
there was a planet to be discovered, which would fill the gap. 

Neptune was then unknown : its distance does not conform to the 
law, being only 300.5. 

224. Discovery of the First Minor Planet. — After Uranus was dis- 
covered in 1 78 1, and its distance had been found to conform to the 
law of Titius, an association of astronomers was formed, to hunt for 
the missing planet between Mars and Jupiter. But the honor of the 
discovery was reserved for Piazzi, of Palermo, who was not a member 
of the association, but was engaged upon a star catalogue. It w T as 
his habit to observe the right ascension and declination of each 
star on several different nights, that he might determine its place 
accuratelv. 



THE ASTEROIDS, OR MINOR PLANETS. I 59 

Upon the evening of the first day of the nineteenth century, he 
observed a list of stars, one of which was destined to bring him 
renown. On January 2d, 3d, and 4th he reobserved the same list, 
and upon comparing his observations, discovered that the thirteenth 
star on the list changed its position from night to night. Here 
at last was a planet, but was it the one sought? For six weeks 
he observed it upon every opportunity, and then he fell ill : when 
he recovered, the planet was so near to the sun that it could no 
longer be found. 

225. Gauss computes the Orbit. — Here was a dilemma. The news 
reached Germany in the early spring, and Gauss, of Gottingen, then 
a rising young mathematician, set himself at the task of finding some 
method of computing its orbit. Finally he discovered the now classic 
method of computing a planet's orbit, when its right ascension and 
declination are known at three different dates. He applied the new 
method to Piazzi's observations, and predicted the future place of 
the planet; at last it was bound by the chains of mathematical 
analysis, more ethereal than a spider's web, but stronger than bronze. 
The association was already hard on the track of the wanderer, and 
when Gauss's results reached them, one of them speedily rediscov- 
ered it, on the last day of the year. Its distance from the sun was 
in fair agreement with the law of Titius, being 25.7 instead of 28. 
It received the name Ceres. 

226. Further Discoveries. — In 1802, 1804, and 1807, Pallas, Juno, 
and Vesta were found. Astronomers were not again successful in 
the quest until 1845, when Astraea was brought to light. Soon after- 
ward the pace of discovery quickened, and now the asteroid hunters 
find them with almost embarrassing rapidity. The family of these 
little strangers is so large, and increasing so rapidly, that the problem 
of taking care of them is becoming a very serious one. The orbit 
of each new one has to be computed, and its place in the sky (right 
ascension and declination) calculated from year to year, so that fresh 
observations can be taken, and a more accurate orbit figured out. 

227. Methods of Search. — Almost all of these bodies are so small 
that they look like fixed stars, and can be detected only by their 
motion. An observer makes a chart of a certain region of the 
sky, containing all the stars visible there through his telescope. 
Then night after night he compares the heavens with his chart. If a 



6o 



DESCRIPTIVE ASTRONOMY. 



5UN 



star-like object not on the chart is seen, its position is carefully noted. 
In a few hours its motion will betray it, if it be a minor planet. 

This process is quite laborious, and is now little used because 
photography has entered the field. A picture of a certain region 
of the sky is taken, the plate being exposed for a couple of hours. 
While the images of stars on the plate are circular, the image of the 
planetoid is a short streak, caused by its motion. 

228. Orbits: Distances: Periods. — The orbits of the planetoids 
are more eccentric than those of the major planets. The orbit 
planes have greater inclinations to the ecliptic, the orbit of Pallas 
being inclined 35 . The mean distances vary from 198,000000 to 
400,000000 miles, the pe- 
riods from three years to 
nine. 

The computations of 
the orbits, as well as of 
the annual ephenieridcs, 
giving the right ascen- 
sion and declination of 
the minor planets, are 
made chiefly by German 
computers. 

The Berliner Jalir- 
bnc]i, issued from the Flg - T 35- 

Imperial Observatory at Kiel, Prussia, contains the results of their 
work. 

The late Prof. Watson 1 left a fund to pay for the twenty-two 
which he discovered, intrusting it to the American National Academy 
of Sciences. 

229. Designations. — Each asteroid has a number, which is usually 
printed thus, @, the numbers being given in the order of discov- 
ery. Names are also given, chosen chiefly from those of female 
divinities in classical mythology. The number of these is about 
exhausted. Some of the unfortunate planetoids have been afflicted 
with such names as Xantippe, Vindobona, and Sophia, to say 
nothing of Walpurga and Chicago. The asteroid last mentioned 

1 James C. Watson, Director of the Observatory at Ann Arbor, Mich., and later of the 
Washburn Observatory at Madison, Wis., author of Watson's Theoretical Astronomy. 




The Zone of Asteroids. 



THE ASTEROIDS, OR MINOR PLANETS. l6l 

was named by the Astronomical Congress which met at Chicago 
during the World's Fair. 

230. Number and Size. — The number is now (1896) over 400. 
Most of them are of quite insignificant dimensions. Vesta, the 
brightest one, is 250 miles in diameter. At opposition it is barely 
visible to a good eye. Ceres, the largest, is nearly 500 miles in 
diameter. The majority are less than fifty miles in diameter. Most 
of the faint ones now being discovered have diameters of only about 
ten miles. Compared with the earth they are as flour-dust to a foot- 
ball. Half a billion of them compacted together might equal the 
earth in bulk. 

231. Atmosphere : Gravity. — The bodies are so small that they 
probably have very rare atmospheres. There are no reliable obser- 
vations bearing on this point. On the assumption that they are 
as dense as the earth, the force of gravity at the surface of one 
of the small ones is about one thousandth part 1 of that which we 
daily contend with. A sharply batted base-ball would leave the 
planet; in a jumping match the spectators could eat lunch while 
waiting for the contestants to come back to terra firma. 

232. Origin. — One view is that they have been formed from a 
larger body by a series of explosions. A single explosion would 
not account for them, for the fragments, after coursing about the 
sun would all return to the point where the explosion occurred. 
After the lapse of ages their orbits would be considerably modified 
by the attractions of the major planets, especially by that of Jupiter. 
But yet the theory of a single explosion is considered untenable 
by those mathematicians who have given particular attention to the 
matter. The present tendency of scientific opinion is to discard 
catastrophes, and to believe in an orderly evolution manifested 
throughout the universe. According to the nebular hypothesis, the 
material composing the minor planets was once collected in a ring- 
surrounding the sun. The ring in condensing formed many planets 
instead of one : the cause of the disruption of the ring ma}' have 
been the powerful attraction of Jupiter. 

1 The principles of mechanics show that for spheres of equal densities the force of 
gravity at their surfaces varies as their diameters. Thus, if two leaden spheres were 
respectively one foot and ten feet in diameter, a grain of sand lying on the surface of 
the latter would be attracted by it ten times as strongly as an equal grain on the 
surface of the former. 

n 



1 62 DESCRIPTIVE ASTRONOMY. 



EXERCISES. 



233. I. If the axis of Mercury is perpendicular to the plane of its 
orbit, and the planet rotates on its axis at a uniform rate from west 
to east once in every revolution, are there alternations of day and 
night at a point on the surface directly between the centres of the 
sun and the planet? 

2. In the case above, would the sun shine on more than half of 
Mercury's surface in 88 days, considering the eccentricity of its 
orbit? 

3. On account of the relative sizes of the sun and Mercury, does 
the former, at each instant, illuminate more than a hemisphere of 
the latter's surface? 

4. Why do Mercury and Venus look black during transit? 

t 5. Mercury at times attains an elongation of 2 8° from the sun. 
If it is at eastern elongation about the last of March, its orbit being 
roughly coincident with the ecliptic, will it be above the celestial 
equator, or below? [In answering this> first fix in mind the position 
of the sun in the ecliptic] 

6. Should a planet, in order to be seen most advantageously 
from your home, be above or below the equator? 
< 7. If Mercury be near its western elongation on April 1, will it be 
in a favorable position to be seen from your home ? 
r 8. If Mercury be near its eastern elongation when school opens 
in the fall, will it be in a favorable position for observation by you ? 
2 9. If you were searching for Mercury with the naked eye, would 
it be more convenient to have it at eastern or western elongation, 
considering only the time of night at which you would look for it? 
} 10. When Venus is morning star, is it east of the sun, or west? 
t 11. Draw two concentric circles of the proper relative sizes to 
represent the orbits of Venus and the earth. Locate the two 
planets upon them, so that Venus shall be at its greatest elongation. 
Then measure the angle of elongation with a protractor. 

12. What phase has Venus when at elongation? 

13. Is Venus gibbous between its superior conjunction and its 
greatest eastern elongation? 

14. Is Venus gibbous between its greatest western elongation 
and superior conjunction? 




MERCURY, MARS, VENUS, THE ASTEROIDS. I 63 

15. Just before inferior conjunction do the cusps of the crescent 
Venus point toward the sun? 

16. Find from the Nautical Almanac when Mercury and Venus 
next reach elongation. 

17. If you have a telescope, make the following test of it and 
your eye. Make on white paper a drawing like Fig. 136. Look 

at it with the telescope, and estimate the 
width of the black drop as compared with 
the diameter of the black circle representing 
the planet. It is well to perform the experi- 
ment out of doors, with your back to the sun, 
so that the paper may be well illuminated. 

18. Why is not a ring of light seen 
around Venus, when it is in transit? 

19. The volumes of two spheres are to each other as the cubes 
of their diameters. The earth is how many times as large as Mars? 

20. Assume that the diameter of the earth is 8,000 miles, and 
that of some asteroid is 10 miles. The earth is how many times as 
large as the asteroid? 

21. Draw two concentric circles of the proper relative diameters 
to represent the orbits of the earth and Mars. (The representation 
will not be very accurate, because Mars's orbit is quite eccentric.) 
Mark a point on the inner circle for the earth, and another on the 
outer to represent the position of the centre of Mars, when in quad- 
rature. Around this centre draw a little circle, to represent the 
planet itself, on an exaggerated scale. Shade one half of this little 
circle, so that it will represent the unilluminated hemisphere of 
Mars. How does the picture show that Mars, as seen from the 
earth, would be gibbous? 

22. Does any planet, as seen from the sun, exhibit phases? 

23. Deimos and Phobos move in the plane of Mars's equator. 
Could an observer at a pole of the planet see them? 

24. Compute the area of the surface of Deimos, assuming it to 
be a sphere six miles in diameter. 

25. Is Bode's Law anything more than a chance coincidence? 

26. The first day of this century was January 1st in what year? 

27. Why does the small size of an asteroid militate against its 
having a dense atmosphere? 



164 DESCRIPTIVE ASTRONOMY. 

CHAPTER X. 

JUPITER. SATURN, URANUS, NEPTUNE. 

" Some displaying 
Enormous liquid plains, and some begirt 
With luminous belts, and floating moons, which took, 
Like them, the features of fair earth." 

Byron. 

234. The Outer Group. — We come now to the consideration of 
the outer group of planets, in comparison with which the planets 
before considered are but pygmies. 

JUPITER, 2/. 

235. Distance and Diameter. — The mean distance of Jupiter from 
the sun is 483,000000 miles; this is five and one fifth times the 
distance of the earth. Its mean diameter is 88,000 miles. The 
planets heretofore considered are nearly spherical, but Jupiter's 
disk as seen in a telescope is a marked oval, showing that the polar 
diameter of the planet is shorter than the equatorial. 1 Jupiter is 
larger than all the other planets put together, being 1,300 times as 
large as the earth. 

236. Revolution and Rotation. — The sidereal period of this planet 
is nearly twelve (11.86) years. Despite its huge bulk, it rotates 
with amazing swiftness, in about 9 h. 55 m. The rotation period 
cannot be obtained with exactness, because, like the sun, different 
parts of the surface rotate in different times, those near the equator 
moving most rapidly. Even a particular feature of his surface does 
not have a uniform rotation time. The time for the great red spot 
(§ 240), for example, slowly increased from year to year during the 
period 1878-86, the total increase being seven seconds. Since 1886 
there has been no change. 

1 The polar diameter is 84,300 miles and the equatorial 89,790 miles. The mean 
diameter is found by adding the polar diameter to twice the equatorial, and dividing 
the sum by 3. 



JUPITER. 165 

237. Appearance to the Naked Eye. — To the naked eye, Jupiter, 
when near opposition, attains a greater brilliancy than any other 
planet, except Venus. At all times, except when too near con- 
junction, it is much brighter than any of the fixed stars. Its light 
is white, with the merest tinge of yellow. Except when near one 
of its stationary points, its planetary nature can be detected in less 
than a week by its motion among the stars. 

Men of extremely acute vision, under the fairest of skies, have 
occasionally seen one of its satellites with the naked eye. One who 
suspects that he sees a satellite should move his head from side to 
side, and see if the suspected satellite moves also ; if it does not, 
it may be a satellite or a fixed star. 

238. Belts. — These are readily discerned with a small telescope. 
The surface seems banded by parallel belts, the most conspicuous 
ones being near the planet's equator. The belts are dark colored. 
In small instruments the dark belts appear of a grayish or brownish 
cast ; but in larger ones a reddish hue is observed. At times they 
even appear pink. The belts are supposed to be rifts in the clouds, 
through which we look down deeper into his atmosphere than else- 
where. The light portions of the disk, as shown in Fig. 137, re- 
semble masses of cumulus cloud. The appearance of the belts is 
sometimes reproduced in the terrestrial clouds lying below the sum- 
mit of Mt. Hamilton, Cal., the home of the Lick observatory. The 
belts change their outlines somewhat from month to month. 

239. Pickering's Theory of the Belts. — Prof. W. H. Pickering, 1 
observing from a high Peruvian table-land, where the atmospheric 
conditions are very fine, says that the appearance of Jupiter is that 
of a uniform white mass of cloud, overlaid by a thin gauze veil of a 
brown material, not unlike our cirrus clouds in structure. This veil 
is more dense in some places than in others ; the dense portions, 
by obscuring the white cloud beneath, cause the dark belts. The 
well known white spots are thought to be due to round or elliptical 
holes in the cirrus layer, through which we see the white surface 
below. He says: "In short, it appears that, were it not for this 
insignificant light gauzy veil of brown cloud, we should find the 
surface of Jupiter, like that of most of the other planets in the solar 
system, almost a perfect blank. This gauzy structure must float in 

1 Of the Harvard College Observatory. 



100 



DESCRIPTIVE ASTRONOMY 




Fig. 



UPITER, AS SEEN WITH THE LlCK TELESCOPE: DRAWN BY KEELER. 



JUPITER. 167 

a nearly transparent atmosphere, surrounding and rising above it ; 
it is this atmosphere which causes the absorption, and which almost 
completely obscures the belts at the limb of the planet." 

240. The Great Red Spot. — This was discovered in 1878; it is 
30,000 miles long and 7,000 broad. It has had various degrees of 
brightness in different years, being almost invisible in 1883 and 
1884, and again in 1892. It is no longer a conspicuous object, even 
in large telescopes. The spot is thought to be a rift in the clouds, 
similar in color to the belts, though usually more vivid. Its cause 
is a mere matter of conjecture. Perhaps it is due to some terrific 
ebullition in the depths of the planet, which causes heated vapors 
to arise and clear away the clouds which would otherwise be above 
it. Its remarkable persistency of form, and its movement (§ 236) 
are against this supposition. 

241. Smaller Spots. — These have been seen frequently of late 
years, and are chiefly either white or black. They are usually 
round or elliptical ; their average size is about that of our moon. 
Perhaps they are cloud masses which are above the general level, 
and therefore show more plainly. Their motions are independent, 
as is shown by the fact that the rotation time of the planet, deter- 
mined by observations of one spot, does not generally agree with 
that found by employing some other spot. 

242. Atmosphere: Spectrum. — All the evidence points to the con- 
clusion that the atmosphere is very extensive. There is no reason 
to believe that any permanent markings have ever been seen on the 
planet, so deep and dense is the enveloping atmosphere. 

The spectroscope gives no certain evidence concerning the com- 
position of the atmosphere, the spectrum of Jupiter being almost 
identical with that of the sun. This shows that the light which 
gives the spectrum is merely reflected sunlight. If the light pene- 
trated to any considerable depth it would suffer marked absorption, 
which would be manifested by bands in the spectrum. There is one 
large faint band in the spectrum, the origin of which is unknown. 

243. Light and Heat. — If Jupiter were hot enough to give out 
light of its own, its moons would not suffer total eclipse when they 
passed into its shadow. But a body may be quite hot without 
being perceptibly luminous, as any disbeliever may discover by 
experimenting with a poker recently withdrawn from the fire. So 



I 68 DESCRIPTIVE ASTRONOMY. 

cloudy is the planet, so great are the changes continually taking 
place on its surface, and so feeble is the action of the sun upon it 
on account of its great distance, that we are compelled to believe 
that the planet is hot. It may be nearly self-luminous, and has 
been called a semi-sun. 

244. Physical Condition. — The average density of the planet is 
only a third greater than that of water. It is therefore chiefly, if 
not entirely, composed of matter in a fluid state ; hot water may be 
one of its chief constituents. We are not to regard it as possessing 
a solid crust, like the earth, but rather as being a seething caldron, 
in which the hot fluids rise from the interior, become cooler, and 
sink back, in a ceaseless round of motion. 

245. The Major Satellites. — These are four in number, and are 
designated in the Nautical Almanac by the Roman numerals I, II, 
III, and IV, according to their distances from the planet, I being 




Fig. 138. — The Orbits of the Major Satellites. 

nearest, and IV the most remote. The smallest, II, is as large as 
the moon, and the largest, III, is nearly as big as Mars. They are 
all visible in a good opera-glass. Their orbits are nearly in the 
plane of the planet's equator. 

246. Eclipses. — Jupiter casts so long and large a shadow away 
from the sun that all the satellites except the fourth suffer eclipse 
once during every revolution. The times of these eclipses are 
given in the Nautical Almanac, and they can be easily observed 
with a small telescope. For obvious reasons the satellite does not 
disappear instantaneously, but fades from sight gradually. 

247. Occultations. — When a satellite, as seen from the earth, is 
behind Jupiter, it has suffered occultation. When it has been just 
disappearing behind the limb of Jupiter, or reappearing, many at- 
tempts have been made to see it shining through Jupiter's atmos- 
phere, with the hope of measuring its refractive power. But most 
of the attempts (even with the Lick telescope) are acknowledged 
failures. 



JUPITER. 



169 



A satellite, when passing between us and the 



in transit. The limb of 



ta t/on 



248. Transits.— 

planet, appears to cross its face and is 
Jupiter is darker than the cen- 
tre of its disk : on this account 
a satellite is visible at the be- 
ginning or end of a transit as 
a bright spot on a dark back- 
ground. When the satellite is 
projected upon some portion 
of the disk which has the same 
brightness and color, it be- 
comes invisible. One occa- 
sionally appears dark, or even 
black at some time during tran- 
sit: this may be because the 
background, on which it is pro- 
jected at the time, is unusually 
bright. The shadows of the 
satellites also make transits : at 
times, a satellite and its shadow 
are seen journeying across Ju- 
piter's face in company. 

249. Markings and Rotation.— 
Many observers have reported 
dark markings on the satellites. Some of the most authoritative 
recent work is that of Professors Schaeberle and Campbell l with 




Fig. 139. — Phenomena of the Satellites. 





Fig. 140. — Markings seen with the Lick Telescope. 

the Lick telescope. Their drawings of the markings on satellite 
III, made with very high magnifying powers, lend strong support 



1 W. W. Campbell, astronomer at the Lick Observatory. 



17O DESCRIPTIVE ASTRONOMY. 

to the theory that it continually keeps the same face toward Jupiter. 
Their observations also show that satellite I is perceptibly elongated, 
and that its long axis points toward the planet's centre. These 
satellites therefore resemble the moon in that their times of revolu- 
tion and rotation are coincident. Schaeberle and others have seen 
a bright polar cap on satellite III. Barnard discovered a white belt 
on satellite I, which causes it to appear double when crossing a 
white part of Jupiter. 

250. Pickering's Observations of the Satellites. — In addition to 
studying the belts, Prof. W. H. Pickering has made careful observa- 
tions of the satellites. He found the first satellite to be at times 
very plainly elliptical, the major axis exceeding the minor by ten 
per cent. For the time of rotation he has deduced a value of 13 h. 
3 m., from a series of observations. 

The rotation time of the second satellite was much more difficult 
to obtain, but a value of over 41 hours was settled upon. 

The observations of the third and fourth satellites favor the 
theory that their times of rotation and revolution are coincident. 

Prof. Pickering's results are in only partial agreement with those 
of other observers : the matter needs much further study. 

251. The Fifth Satellite. — On Sept. 9, 1892, Dr. E. E. Barnard 
discovered a fifth satellite, with the Lick telescope. It is a tiny 

point of light, which can be observed with the 
most powerful telescopes only. Its distance 
from the centre of Jupiter is 112,000 miles and 
its time of revolution 11 h. 57 m. 22.56 sec. Its 
Fig. 141.— Jupiter and diameter is estimated as 100 miles. If it were 
the Orbit of the a f ew thousand miles nearer to the planet, it 
would probably be torn in pieces by the attrac- 
tion of the latter, which would be much more powerful upon the side 
of the satellite turned toward it at any time than on the opposite side. 

252. Velocity of Light. — The fact that light does not travel from 
one point to another instantaneously was discovered in 1675 by 
Roemer, a Danish astronomer. The discovery was made by the 
discussion of observations of Jupiter's satellites. The eclipses recur 
at nearly equal intervals. By noting the times of the eclipses of 
satellite I, for example, during the period of one revolution of 
Jupiter about the sun, one can calculate with great accuracy the 




JUPITER, SATURN. IJl 

average interval of time between two successive eclipses : the 
satellite suffers some 2,500 eclipses during that period. When 
Jupiter is in opposition, let the time of an eclipse be noted : by 
means of the known interval between two successive eclipses, com- 
pute the day, hour, and minute when an eclipse will occur three 
months after opposition, at which time the earth will be farther 
from Jupiter. The eclipse will happen later than the predicted time. 
Predict another eclipse near the time of the planet's conjunction with 
the sun, when the earth is 186,000000 miles farther from Jupiter than 
at opposition. The eclipse will again be behindhand, and by a larger 
amount than before. Roemer sagaciously guessed that the eclipse 
which took place near the time of conjunction really happened on 
time, but that the light which brought the message to the observer 
took time to cross the extra distance of 186,000000 miles. In this he 
was right : the time required for light to cross the earth's orbit is close 
to 1 ,000 seconds. 

SATURN, h. 

253. Distance and Diameter. — The distance of this most enchant- 
ing of planets from the sun is 886,000000 miles. Its mean diameter 
is 74,000 miles. 

254. Revolution and Rotation. — The sidereal period is 2o,}4 
years. The rotation time is hard to determine because so few 
small well defined spots have ever been seen on its surface. 
Prof. Asaph Hall has derived a value of 10 h. 14 m. 23. 8 sec. from 
his observations of a white spot which appeared on the ball in 
December, 1876, and was visible for a month. 

255. Appearance to the Naked Eye. — Because of its greater dis- 
tance, Saturn is much fainter than Jupiter. It alone of the planets 
has a decided yellowish tint ; it is generally as bright as a first 
magnitude star, and may be distinguished from a star by the fact 
that it does not twinkle. All planets have this peculiarity except 
when near the horizon. A person who is acquainted with the con- 
stellations may find it easily by looking up its right ascension and 
declination in the Nautical Almanac, and locating it on a star map. 

256. Telescopic Appearance. — The first view of Saturn with a 
large telescope usually calls forth an exclamation of wonder and 
delight. For the globe is seen to be surrounded by a marvellous 



172 



DESCRIPTIVE ASTRONOMY. 



ring system, which is unique in the solar system, and, so far as 
we know, in the entire universe. A goodly retinue of satellites is 
also seen attending this majestic orb. 

The ball is encircled by rather bright belts near the equator, 
and by fainter ones at higher latitudes. These belts are not sub- 
ject to much change of appearance, except that due to imperfect 
seeing caused by the fluctuations of our own atmosphere. Serenity 
is natural for the oldest of the gods. 




Fig. 142. — Saturn, as seen with the Lick Telescope: Drawn by Keeler. 

257. Discovery of the Rings. — In 1610 Galileo discovered that 
the planet appeared triform. This was due to the imperfection of 
his telescope. 

He said that Saturn had two servants who aided him on his way. 
Great was his perplexity and chagrin to find that the attendants 
disappeared after a year or two. In a letter to a friend he said : — 

"What is to be said concerning so strange a metamorphosis? Are the 
two lesser stars consumed after the manner of solar spots? Have they 
vanished, or suddenly fled? Has Saturn, perhaps, devoured his own 
children? Or were the appearances indeed illusion and fraud, with 
which the glasses have so long deceived me, as well as many others to 
whom I have shown them? . . . The shortness of the time, the unex- 
pected nature of the event, the weakness of my understanding, have 
greatly confounded me." 



SATURN. 173 

Nevertheless, in the latter part of the letter he ventures to predict 
that the lost bodies will reappear ; and he himself, as we learn from 
a later letter, saw them again as " ears," one on each side of the 
central ball. Forty odd years later, Huyghens, a Dutch astronomer, 
advanced the theory that Saturn had a ring, announcing it in the 
form " aaaaaaa ccccc d eeeee g h iiiiiii 1111 mm nnnnnnnnn 0000 
pp q rr s ttttt uuuuu." These letters, properly arranged, form 
the Latin sentence, Annulo cingitur, tcniii, piano, nusqnam cohae- 
rcjitc, ad eclipticam inclinato. The translation is : " It is encircled 
by a thin flat ring, nowhere touching, inclined to the ecliptic." 

258. Divisions and Dimensions of the Ring System. — The ring 
announced by Huyghens has been found with powerful telescopes 
to be composed of three, two of which are bright and the third 
dark. They are shown clearly in Fig. 142. The division between 
the outer ring and the middle one has been named Cassini's di- 
vision, after the Italian astronomer who first noticed it, twenty years 
after Huyghens's announcement; it is about 2,200 miles in width. 
There is a finer division in the outer ring, known as the Encke 
division. Many others have been suspected. One has been 
found by Keeler 2 at the Lick Observatory. 

The inner ring is much darker and fainter than the others ; it is 
known as the dark or dusky ring. The extreme diameter of the 
ring system is 173,000 miles, and its breadth is about equal to the 
semidiameter of the ball. The rings are not of uniform thickness, 
as is shown when the edge of the system is turned toward us. The 
average thickness is not far from one hundred miles. 

259. Disappearance of the Rings. — This phenomenon, which was 
so sore a trial to Galileo, is explained by Fig. 143. The plane 
of the rings coincides with that of the planet's equator, but is in- 
clined 27 to the plane of the planet's orbit. Saturn keeps the 
successive positions of its rings parallel to each other in its journey 
around the sun. As the plane of the earth's equator passes through 
the sun in March and September, so the plane of Saturn's rings 
passes through the sun twice in one of its revolutions. Since the 
earth, as seen from Saturn, is close to the sun, the plane of the 
rings will pass through the earth within a few weeks of the same 
time. At such a time, the rings disappear because they are thin, 

1 James E. Keeler, Director of the Allegheny Observatory. 



174 DESCRIPTIVE ASTRONOMY. 

and edgewise to us. The disappearances happen at intervals of 
fifteen years : at times midway between them the rings are seen 
most favorably. 




Fig. 143. — Different Positions of the Rings. 

260. The Dark Ring. --- This ring is sometimes called the crape 
or gauze ring, because it is semi-transparent. Dr. Barnard, at the 
Lick telescope, on Nov. r, 1889, observed an eclipse of one of the 
satellites. After emerging from the shadow of the ball it recovered 
its normal brightness, and soon plunged into the shadow of the 
dusky ring; it then became fainter and fainter, but did not disap- 
pear until it had passed through the shadow of the crape ring and 
into the shadow of the inner one of the two bright rings; then it dis- 
appeared entirely. This shows that sunlight sifts through the dark 
ring, and that the transparency of the latter decreases regularly 
from its inner edge to its outer, where it joins the inner bright ring. 

261. Structure of the Rings. — The researches of mathematicians 
have demonstrated that neither ring can be an unbroken mass, either 
solid or liquid. In either case the ring would have been destroyed 
long ago by the attraction of the ball. The hypothesis now 
adopted is that it is composed of myriads of minute bodies, a con- 
geries of closely packed moons, each of which has an orbit of its 
own. In the dark ring the bodies are less closely packed together 
than in the bright ones. At the outer edge of the dark ring they 
are thought to be more densely crowded than at its inner edge. 



SATURN. 



'75 



This hypothesis concerning the structure of the rings has been 
confirmed by observation. The separate particles are much too 
small to be seen separately, but their existence did not escape the 
spectroscope in the hands of Keeler in April, 1895. If the bright 
ring were solid, its outer edge would travel faster than the inner, 
just as a point on a tooth of a circular saw moves more swiftly than 
one nearer the centre. On the other hand, if the ring is made up 
of moonlets, those near its outer edge must move with less velocity 
than those near the inner. Professor Keeler's beautiful photo- 
graphs of the spectrum of the bright ring showed that the outer and 
inner edges had respectively velocities of 10. 1 and 12.4 miles per 
second. These values agree well with theoretical ones computed 
according to Kepler's Laws. 

262. Stability of the System. — The Cassini division is supposed 
to be due to the attraction of the largest satellite, which has changed 
the orbits of the bodies which once occupied 
the division. The outer divisions were presum- 
ably caused in the same manner. 

These minute bodies must be continually 
colliding with each other, so that some of them 
lose velocity, and are drawn into smaller orbits Fi s- 144- — Old Draw. 

. . . r n ' i 11 mi ING OF Saturn. 

by the attraction of the ball. lhe appearance 

of the dark ring suggests that the ring system is being thus 
disintegrated. A comparison of the old drawing shown in Fig. 144 
with Fig. 142 indicates that the space between the ball and the 
inner edge of the rings is now smaller than formerly. All these 
considerations have led to the hypothesis that Saturn is indeed 
" devouring his children." 

However, no evidence of such a change is given by accurate 
measures of the dimensions of the ring made during the past one 
hundred years. 

263. The Satellites. — These are eight in number. Japetus, the 
outermost, is at a distance of 2,212000 miles, and occupies seventy- 
nine days in a revolution. Mimas, the innermost, is only 30,000 
miles beyond the outer edge of the ring system, and completes its 
circuit in less than a day. Titan, the largest, is nearly as big as 
Mercury. All move in the plane of the rings excepting Japetus, 
the orbit of which is inclined io° to it. Japetus suffers remark- 



o 



I 76 DESCRIPTIVE ASTRONOMY. 

able and regular variations in brightness, which are explained by 
assuming that one hemisphere of it is much brighter than the 
other, and that it always presents the same face to the planet, as 
our moon does to the earth. 

264. Physical Condition of the Planet. — The mean density of 
the ball is less than that of water, or even alcohol, closely agreeing 
with that of ether. The cloud shell surrounding the kernel of the 
planet is so deep that it hides beneath its placid exterior nearly all 
the commotions which are taking place. The central nucleus 
seems to possess heat sufficient to maintain this cloud mantle, but 
not sufficient to give rise to such activity as Jupiter manifests. 
The spectrum of the ball is that of the sunlight reflected from its 
surface, with the addition of some dark bands caused by the 
absorption of the sunlight by an unknown constituent of the 
planet's atmosphere. The spectrum of the rings contains no 
absorption bands. 

URANUS, S or #. 

265. Discovery. 1 — William Herschel discovered Uranus in 
March, 178 1. He was an organist at Bath, England, — a man of 
no mean musical attainments. In studying the mathematical 
theory of music, he had occasion to enlarge his knowledge of 
mathematics; from this he was led to optics, and became exceed- 
ingly interested in telescopes and astronomy. He resolved to 
make a reflecting telescope : supporting himself by his profession, 
he devoted his leisure to grinding and polishing specula and 
lenses. Rushing home from a concert, he would plunge at once 
into work on his mirrors, without even stopping to take off his 
lace ruffles. Mirror after mirror was constructed, put to use, and 
laid aside or sold, each giving place to a new one, more perfect, or 
of larger size. When engaged in putting the finishing touches on 
one of his great mirrors, he often sat at his work for hours, food 
being put into his mouth by his devoted sister Caroline, who sat 
by his side, and beguiled the time by reading "The Arabian 
Nights." 

1 This article is chiefly a condensation of material found in Ball's "Story of the 
Heavens." 



URANUS. 177 

Such unremitting enthusiasm and genius must find their reward. 
After half a dozen years of this assiduous toil, he succeeded in 
constructing a seven-inch reflector of exquisite optical perfection. 
He resolved to examine all the stars above a certain order of 
brightness. Now a fixed star is the merest point of light; the 
more perfect the telescope, the smaller is the image of the star. 

Star after star passed in review before him. Finally, on the 
night of March 13, 1781, he perceived an object which looked like 




Fig. 145. — Sir William Herschel. 

a star, except that its disk was a trifle larger than that of a star of 
the same brightness. Many a time had this object been observed 
by other astronomers, but they had noticed no peculiarity in its 
appearance. Herschel soon found that it was in slow motion; he 
reasoned that it must be nearer than the fixed stars, and not 
dreaming that he had discovered a new major planet, the others 
having been known since the dawn of astronomical science, an- 
nounced that he had found a comet. Astronomers at once set to 



I70 DESCRIPTIVE ASTRONOMY. 

work observing it. Computations of its orbit followed. Within 
a year the mathematicians had demonstrated that the orbit was 
nearly a circle, twice as large as the path of Saturn. The object 
was therefore a planet. Herschel proposed the name Georgium 
Sidus in honor of his sovereign; Laplace suggested the designa- 
tion Herschel. The name finally adopted was proposed by Bode. 
This notable extension of the confines of the solar system was 
hailed with the greatest enthusiasm. King George knighted 
Herschel, and gave him ^200 a year. The further career of Her- 
schel, who finally constructed a reflector four feet in aperture and 
forty feet in focal length, stamps him as foremost among astronom- 
ical observers. 

266. Distance and Diameter. — The distance of Uranus from the 
sun is 1,782,000000 miles. Its diameter is 32,000 miles, four 
times that of the earth. 

267. Revolution and Rotation. — ■ The sidereal period is eighty- 
four years. The time of rotation is unknown, because no suffi- 
ciently definite markings have ever been seen on its surface. 

268. Appearance. — To the naked eye it appears as a small star 
just on the limit of visibility. It may be found by the use of the 
Nautical Almanac, which gives its right ascension and declination 
throughout the year. 

In a large telescope it exhibits a greenish disk occasionally 
marked by faint belts. Granting, as mathematical theory de- 
mands, that the planes of the orbits of the satellites nearly coin- 
cide with that of the planet's equator, the belts are not parallel to 
the equator, an unexplained anomaly. 

269. The Satellites. — Uranus is attended by four of these bodies, 
no one of which can be seen by an ordinary eve with a telescope 
less than eight inches in aperture. The diameter of the largest is 
probably five hundred miles. Their orbits lie in one plane, which, 
strange to say, is nearly perpendicular to the ecliptic. They also 
revolve backwards, that is from east to west, in their orbits, unlike 
any other satellites before considered. 

270. Physical Condition. — Of this little is known. The spec- 
trum of the planet exhibits some conspicuous bands, thought to be 
due to the absorption of a dense atmosphere. Sunlight at Uranus 
being only gl^ as intense as at the earth, the processes of cloud 



NEPTUNE. 179 

formation must be dependent chiefly on internal heat. Its mean 
density is less than that of bituminous coal. 

NEPTUNE, t£. 

271. Discovery. — The discovery of Neptune is esteemed the 
most notable triumph of mathematical astronomy. It was no mere 
accident, nor was it brought about simply by a diligent search with 
the telescope. Forty years after the discovery of Uranus, Bou- 
vard, a French astronomer, published tables of its motion, by 
means of which its place could be predicted for the future. But 
the planet refused to follow the path marked out for it; farther 
and farther it departed from the appointed course. In twenty years 
the discrepancy between theory and observation had become intol- 
erable. To be sure, the difference could not yet be perceived by 
the naked eye, but the unfailing accuracy of the observations 
loudly proclaimed that there was some fault in the theory of the 
planet's motion. Was the law of gravitation partially inoperative 
at this enormous distance from the sun ? Had a flaw been found 
at last in the marvellous researches of Newton ? By no means. 
From many quarters came the suggestion that some unknown body 
was displacing Uranus by its powerful attraction. But could the 
position of the troublesome stranger be pointed out? 

John Couch Adams, a tutor in the University of Cambridge, 
England, grappled with the problem. In October, 1845, ne com- 
municated to the Astronomer Royal of England the elements 
of the orbit of the suspected planet, together with a prediction of 
its place in the sky. But the Astronomer Royal : did not regard 
these investigations of a young and comparatively unknown man 
as entitled to much confidence. He however called the attention 
of a few of his friends to them, and wrote Adams asking some 
further information : no reply reached him. He therefore pigeon- 
holed the manuscript. One of the friends wrote to Lassell, who 
possessed a fine two-foot reflector which was mounted near Liver- 
pool, begging him to search for the planet. But Lassell was 
suffering from a sprained ankle, and when he recovered, the letter 
was nowhere to be found, and the telescopic search was not made. 

1 Sir George Biddell Airy. 



i8o 



DESCRIPTIVE ASTRONOMY. 



Meanwhile Leverrier, a brilliant French astronomer, likewise a 
young man, had employed his powers upon the same problem. 
On June I, 1846, he sent a communication to the French Academy 
of Sciences giving the direction in which the planet was to be 
found. 




Fig. 146. — John Couch Adams. 



The English astronomers, finding that Leverrier's results agreed 
with those of Adams, awoke from their lethargy, and began to 



NEPTUNE. l8l 

bestir themselves. Professor Challis, the astronomer of the 
University of Cambridge, commenced a search. Doubting the 
accuracy of the predictions, he began to map a large area of the sky, 
hoping by comparison of maps of the same region made on different 
nights to detect the planet by its change of position if it were 
really there. 

Sir John Herschel (son of Sir William), in a public address, said 
concerning the unknown body: "We see it as Columbus saw 
America from the coast of Spain. Its movements have been felt, 
trembling along the far-reaching line of our analysis, with a cer- 
tainty hardly inferior to that of ocular demonstration." 

Three times Challis observed the planet, but did not look sharply 
enough to notice its disk, which was larger than that of the stars. 
While he was laboriously heaping up observations and neglect- 
ing to compare them, the prize of discovery slipped from his grasp. 
Leverrier had written to Galle, of Berlin, where some excellent 
star charts were being made, asking him to direct his telescope to 
a certain point on the ecliptic, and saying that he would find 
within a degree of that point a new planet, as bright as a star of 
the ninth magnitude (§ i) and having a perceptible disk. Galle 
did as he was bidden, and found the planet within half an hour, on 
Sept. 23, 1846. Success is to the confident. 

272. Distance and Diameter. — ■ The mean distance of Neptune 
from the sun is 2,792,000000 miles. It is therefore a billion 
miles farther than Uranus, and thirty times as far as the earth. 
The diameter is 35,000 miles. 

273. Revolution and Rotation. — The sidereal period is nearly 165 
years. The time of rotation is unknown, because no well defined 
spots have ever been seen on the surface. 

274. Appearance. — Neptune is too faint to be visible to the 
naked eye. A good opera-glass will show it. It may be found by 
using the Nautical Almanac and a star map, as formerly explained 
(§ 255). In a large telescope its greenish disk is readily perceived, 
but no marks have been seen upon it. 

275. Satellite. — There is one satellite, a very faint object, sup- 
posed to be of the size of our moon. The plane of its orbit is in- 
clined 35 to the ecliptic, and the satellite, like those of Uranus, 
moves backwards from east to west. 



1 82 DESCRIPTIVE ASTRONOMY. 

276. Physical Condition. — The spectrum is similar to that of 
Uranus, showing faintly the same absorption bands, which are 
presumably due to a dense atmosphere. The sunlight is only 9-^ 
as intense as at the earth; perhaps no cheering ray of sunlight 
penetrates the clouds in which the planet is entirely enveloped. 
The density of Neptune is a little less than that of Uranus. The 
two planets are almost identical in size and general make up. 

277. Planets beyond Neptune. — Such planets have been suspected 
on various insufficient grounds ; they have been hunted for with 
large telescopes, both visually and by means of photography, which 
brings to light stars too faint to be seen with the most powerful 
telescopes. Xo success has yet attended these efforts. The 
24-inch Bruce photographic telescope, 1 if used for long exposure 
photographs in the vicinity of the ecliptic, would reveal hosts of 
new asteroids, and might bring to notice ultra-Neptunian planets. 

EXERCISES. 

278. 1. Why is Jupiter's disk elliptical? 

2. The volumes of spheres are to each other as the cubes of their 
radii or diameters. Verify the statement that Jupiter is 1,300 times 
as large as the earth. 

3. What reasons are there for thinking that Jupiter has no solid 
crust ? 

4. Though Jupiter is 1, 300 times as large as the earth, its density 
is only 0.24 as great. Its mass is therefore how many times that 
of the earth ? 

5. Ought Jupiter to appear gibbous, when at quadrature, like 
Mars ? 

6. Why do Jupiter's belts, if they are due to the absence of 
clouds, look darker than the cloudy portions ? 

7. If the interior of Jupiter were so hot as to shine through his 
atmosphere, would the spectrum be continuous, or crossed by dark 
lines ? 

8. Can one of Jupiter's satellites be in occultation and also in 
eclipse at the same time ? 

1 By far the most powerful telescopic camera in existence, — the property of Harvard 
College Observatory. 



JUPITER, SATURN, URANUS, NEPTUNE. 1 83 

9. (a) If one of Jupiter's satellites were in transit, and were 
almost exactly between us and its own shadow, would Jupiter be 
near opposition or near quadrature? 

(b) Might it be near conjunction ? 

10. (a) Why is an eclipse of one of Jupiter's satellites not instan- 
taneous ? 

(b) Is an occultation instantaneous? 

n. If Jupiter's atmosphere were sufficiently transparent to let 
the light of a satellite through when it was disappearing in occulta- 
tion, would the time of disappearance be delayed? 

12. Would the refractive power of Jupiter's atmosphere delay 
the time at which a satellite entered upon a transit over his 
disk? 

13. Our moon, when eclipsed, is usually visible. Why are not 
eclipsed satellites of Jupiter similarly visible? 

14. Suppose that the shadow of one of Jupiter's satellites, when 
moving across its disk, fell upon some portion that was decidedly 
brighter than the average, would the shadow look darker in conse- 
quence, or lighter? 

15. A person on Jupiter, in the shadow of one of its satellites, 
would see an eclipse of what ? 

16. If satellite I was once, or is now, a fluid mass, why is it 
elongated ? 

17. If satellite I was once fluid, and rotated more swiftly than 
now, what force has checked its velocity of rotation ? 

18. If satellites III and IV were fluid, being of the same size 
and composition, why should III be more elongated than IV? 

19. What is the velocity of light, in miles per second, on the 
assumption that light takes just 1,000 seconds to cross the earth's 
orbit? 

20. If Jupiter's fifth satellite has the same albedo as satellite 
IV, that is, reflects sunlight just as well, state two reasons why it 
is difficult to see. 

21. The volume of Saturn is how many times that of the earth, 
if its diameter is nine times as great? 

22. The earth's diameter being taken as unity, the diameters 
of the other planets are roughly as follows: Mercury J, Venus 1,' 
Mars |, Jupiter 11, Saturn 9, Uranus 4, Neptune 4.]. Show that 



184 DESCRIPTIVE ASTRONOMY. 

the volume of Jupiter is greater than that of all the other major 
planets together. 

23. Assuming the approximate data in the preceding exercise, 
find whether the surface of Jupiter is as great as the combined 
surfaces of the other major planets. 

24. Could we ever see an occupation of Mars by Jupiter? 

25. Why did Galileo's two attendants of Saturn disappear? 

26. What appearance in Fig. 142 shows that the dark ring of 
Saturn is transparent ? 

27. Is the shadow of the ball of Saturn, as cast upon the rings, 
visible in Fig. 142 ? 

28. Is the shadow of the bright rings of Saturn, cast upon the 
ball, visible in Fig. 142 ? 

29. Does the plane of Saturn's rings, when extended indefinitely, 
ever pass between the earth and the sun ? 

30. Does the sun ever illuminate both sides of Saturn's rings at 
the same time ? 

■31. Is one side of Saturn's rings perpetually unilluminated by 
the sun? 

32. If the plane of the rings ever passed between the sun and 
the earth, could we then see the bright side of the rings ? 

33. Suppose that the ball of Saturn was a perfect sphere of 
uniform density throughout ; also that the ring system was a solid 
sheet of matter, truly circular, uniform in both thickness and 
density, and concentric with the ball ; suppose further that one of 
the satellites attracting the ring pulled it to one side, so that it 
was no longer concentric with the ball, would the attraction of the 
ball pull it farther until it struck the surface of the ball? 

34. Would a great difference in brightness between the outer 
edge of Saturn's dusky ring and the inner edge of the bright ring 
next to it militate against the theory advanced in § 262, that Saturn 
is " devouring his children " ? 

35. What does the absence of absorption bands from the spec- 
trum of the rings indicate concerning their atmosphere? 

36. Why is the direction east to west called backwards in 

§269? 

37. How can the statement in § 276, that sunlight at Neptune 
is only -g-J-g- as intense as at the earth, be figured out from the 



JUPITER, SATURN, URANUS, NEPTUNE. 1 85 

statement in § 272, that Neptune's distance is thirty times that of 
the earth? 

38. Ought Neptune to look very gibbous when at quadrature? 

39. When Neptune is at opposition, show that the light by 
which we see it left the sun 8 h. 1 1 m. 40 sec. before it reached us. 
Assume that light takes five hundred seconds to come from the sun 
to the earth, and that Neptune's distance from the sun is thirty 
times ours. 

40. Sunlight at Neptune would be how many times as intense 
as our moonlight ? (§ 165.) 

41. (a) Does Neptune disturb the motion of Mercury at all? 
(b) Does Mercury disturb that of Neptune ? 



J 86 DESCRIPTIVE ASTRONOMY. 



CHAPTER XL 



COMETS AND METEORS. 

" Stranger of heaven, I bid thee hail ! 
Shred from the pall of glory riven, 
That flashest in celestial gale, 
Broad pennon of the King of Heaven ! " 

Hogg. 

279. Comets in General. — The word i( comet " is derived from a 
Greek word, which means the long-haired one; the designation 
evidently came from the resemblance of the tail to dishevelled 
tresses. These bodies are very different in behavior from the 
staid and trusty planets. They usually come unheralded, change 
their form and brightness from night to night, display all their 
antics in a few weeks or months, and are off again, perchance to 
whisk about some other world in like gay fashion. 

280. Discovery. — In the early ages only those comets were 
discovered which were bright enough to be conspicuous to the 
naked eye. But of late years a comet does not often become 
visible to the naked eye before one of the comet-hunters 1 has 
detected it with his telescope. 

These observers usually employ small telescopes equipped with 
low powers, so that the field of view may be large. Hour after 
hour they scan the face of the sky, hunting for nebulous-looking 

1 The following extract about Messier, a comet-hunter of the eighteenth century, is 
taken from Langley's New Astronomy ; it is given there as a translation from Delambre's 
History of Astronomy : " He has passed his life in nosing out the tracks of comets. 
He is a very worthy man, with the simplicity of a baby. Some years ago he lost his 
wife, and his attention to her prevented him from discovering a comet he was on the 
search for, and which Montaigne of Limoges got away from him. He was in despair. 
When he was condoled with on the loss he had met, he replied, with his head full of 
the comet, ' Oh dear ! to think that when I had discovered twelve, this Montaigne 
should have got my thirteenth ! ' And his eyes filled with tears, till, remembering what 
it was he ought to be weeping for, he moaned, ' Oh my poor wife ! ' but went on crying 
for his comet." 



COMETS AND METEORS. 1 87 

objects. Faint comets ordinarily look so much like nebulae that 
they cannot be distinguished from them, except by their motion. 
A comet-hunter, finding such an object, looks at his catalogue of 
nebulae to see if it is given there. If not, it may be a comet, 
and he watches it until he has found out whether it is in motion ; 
if in motion, he announces it as a comet. 

Photography has now scored its first success in this field. 
Dr. Barnard was the first to discover a comet by photography. ] 
Special photographic lenses are employed which enable the 
astronomer to photograph on one plate a large region of the 
sky. One drawback to this method is that the exposure times are 
necessarily long, since a faint object does not impress itself on 
the plate quickly. 

281. Number : Designation. — During the past three thousand 
years there have been recorded about seven hundred of these bod- 
ies. Before the invention of the telescope the rate of discovery was 
slow, because only a few comets are conspicuous objects to the 
naked eye. At present about half a dozen are found annually, 
the majority of them being merely telescopic, i. e. too faint to be 
seen without a telescope. 

There may be thousands of comets which never come near 
enough to the earth to be discovered. It has been estimated that 
millions of them never come nearer to the sun than Neptune does. 
Kepler thought comets to be as numerous in the heavens as fishes 
in the ocean. 

Comets especially noteworthy receive special names. The great 
comet of 1858 received the name of Donati's comet, Donati being 
its discoverer. Encke's comet was named for him, because he 
made some striking researches concerning its movements. The 
wonderful comet found by Finlay, at the Cape of Good Hope, in 
the fall of 1882, was so majestic that no man's name has been 
attached to it. It is known as "The Great Comet of 18S2. " 

Other designations are used for the convenience of astronomers. 
Comet a 1892 denotes the first comet discovered in that year: 
Comet / would be the sixth comet. Roman numerals are also 

1 So faint was the photographic impression that, when another keen-sighted astrono- 
mer was asked to find the comet on the plate, he was unable to do so, though he 
succeeded in seeing it after it was pointed out to him. 



1 88 DESCRIPTIVE ASTRONOMY. 

used to denote the order of arrival at perihelion. Comet 1889 V. 
was the fifth in that year to arrive at its perihelion. 

282. Brightness and Visibility. — Comets vary greatly in bright- 
ness, some being so faint that only a powerful telescope reveals 
them, others being so brilliant that they can be seen in full day- 
light though close to the sun. The brightness continually changes 
as the distances of the comet from the earth and sun change. But 
there are also irregular fluctuations of brightness. 

There is rarely a week through the year when some comet is not 
in sight. Some of them are seen only a few weeks before they 
become too faint to be observed longer. But the large telescopes 
scattered throughout the world now enable astronomers to follow 
comets for a much longer time than before. One comet was seen 
with the Lick telescope over two years after its discovery. 

283. Parts of a Comet. — Three parts of a comet are usually men- 
tioned, — the coma, or head, the nucleus, and the tail. 

The coma is the cloudlike form, which is the distinguishing 
mark of a comet. Faint comets are frequently all coma, no tail 
or nucleus being visible. 

The nucleus is a starlike or planetary point in the coma, the 
most condensed portion of the comet. It is likewise the most 
brilliant part, and usually contains most of the comet's mass. 

The tail is the train of tenuous matter which, streaming from 
the head, is the chief glory, to the naked eye, of a large comet. 

284. Forms of Orbits. — If a right triangle be revolved about 
one of its perpendicular sides as an axis, it will generate a cone. 
The base of the cone will be a circle (Fig. 147). A section CD of 
the cone, made by a plane parallel to the base, is a circle. If the 
cutting plane is inclined to the base by a less angle than VAB, the 
section EM is an ellipse. When the cutting plane FGH is parallel 
to VA, the section is a parabola. A plane IKL, which is more 
nearly parallel to the axis VO than FGH was, cuts an hyperbola 
out of the conical surface. The circle and ellipse are closed 
curves, but the parabola and hyperbola are not. 

The three curves are delineated in Fig. 148. The branches of 
a parabola become more nearly parallel to each other the farther 
they extend. The branches of an hyperbola continually diverge. 
A comet which travels in an ellipse will return to our vision at 



COMETS AND METEORS. 



189 



regular periods, unless it undergoes some powerful disturbance, 
which alters its path. A parabolic or hyperbolic comet never 
returns. 




L H 

Fig. 147. — Conic Sections. 




Fig. 148. — Varieties of Orbits. 



285. Significance of these Forms. — Suppose that a small body 
is at a very great distance from the sun, and both bodies are 
motionless. The body will begin to fall toward the sun, its path 
being a straight line directed toward the sun's centre. Another 
small body, likewise at a distance practically infinite, has a slight 
motion of its own, but is not moving directly toward the sun ; 
urged on by the sun's imperious attraction, its velocity will contin- 
ually increase; however, as it is not going directly toward the sun, 
it will not strike it, but as it goes past the pull of the sun will 
cause its path to be violently curved ; whirling around the sun, it 
will return toward the infinite depths of space from which it came ; 
its orbit is a parabola. A body which has originally a very con- 
siderable velocity of its own will come down to the sun in an 
hyperbolic orbit, and then retreat, never again to visit us. 

A body moving in a parabola may have its velocity checked, as 
it approaches the sun, by the attraction of some planet : its orbit 
will thus be changed to an ellipse. Were the movement of the 
body accelerated by the planet's action, the orbit would become an 
hyperbola. 



I 90 DESCRIPTIVE ASTRONOMY. 

286. Groups of Comets. — A comparison of the orbits of comets 
shows that there are certain groups of them, pursuing nearly the 
same paths. The Great Comet of 1882 belonged to one of these 
groups, the orbits of those of 1668, 1843, 1880, and 1887 being ver y 
similar to its orbit. Since each of these four comets approaches 
very close to the sun's surface, when at perihelion, a startling 
theory was promulgated in 1882 that these were one and the same, 
the periodic time being continually lessened by passing through the 
sun's atmosphere, and that the comet would plunge into the sun 
in a few months. It is needless to say that this anticipation was 
not verified. Tisserand 1 has recently shown that comet groups may 
be caused by the disruption of the nucleus of a single comet, in 
consequence of the heat or tidal action of the sun. The fragments 
would thereafter pursue very similar orbits, the chief difference 
being in the periodic times. A comet moving in an ellipse is 
called & periodic comet, because it returns at regular intervals. 

287. Planetary Families of Comets. — Fig. 149 shows the orbits of 
a number of recent periodic comets. Inspection shows that the 
aphelion of each orbit lies near the orbit of Jupiter. This fact 
suggests that Jupiter is the planet which retarded these comets 
as they were sweeping down towards the sun from interstellar 
space, and transformed their orbits into ellipses. 

Jupiter, having a much more powerful attractive force than any 
other planet, is credited with a larger family of comets, about twenty 
individuals in all. Neptune, the outpost of the solar system, has. 
succeeded in capturing half a dozen. 

It should be borne in mind that a planet's influence does not 
always work in favor of a comet's capture. The planet may be 
so placed with reference to the comet as to accelerate its motion. 

288. Successive Changes in Orbits : Exact Parabolas. — A comet 
which approaches near a planet and suffers a change of orbit may 
come near it again after a few years and suffer another change. 
An ellipse may be changed into another ellipse, in which the 
comet revolves in a longer or shorter period than formerly. A 
number of such instances are known. Comet 1889 V. (see §292) 
was in 1884 revolving in an ellipse having a period of about 
thirty years. In 1886 it came so near to Jupiter that it was 

1 Director of the Paris Observatory. 



COMETS AND METEORS. 



I 9 I 



for a short time almost dominated by that planet, and described 
an hyperbola about it; when it finally escaped from Jupiter, and 
yielded to the power of the sun again, its period was only seven 
years. In 192 1 Jupiter will modify its orbit seriously again, prob- 
ably enlarging it greatly. The ellipse may become a parabola or 
even an hyperbola. When this is said, it must be remembered 




Fig. 149. — Orbits of some Comets of Jupiter's Family. 

that an orbit is called a parabola when its form approaches that 
curve so nearly that our observations detect no appreciable devia- 
tion from it. The chances are that no orbit is an exact parabola, 
for if a comet were moving at any instant in a parabolic orbit, 
the slightest attraction from any body (it must suffer many such 
pulls) would change the orbit into an ellipse of very long period, 
or an hyperbola. The orbits of most comets are sensibly parabolic. 
289. Changes of Appearance. — When a telescopic comet is first 
seen, it appears, as has been said, like a filmy cloud on the bosom 



192 DESCRIPTIVE ASTRONOMY. 

of the. sky. The coma usually looks more condensed toward the 
centre. As it approaches the sun, it grows brighter, and the 
nucleus, if it has any, comes into view, like a blurred star shining 
through a mass of foggy light. 

As it is warmed by the sun signs of activity become manifest. 
The tail gradually forms, increasing in size and splendor as the 
comet comes nearer the sun. The nucleus seems to be in ebulli- 
tion, throwing off masses of vapor, or ejecting jets. After peri- 
helion passage the nucleus gradually becomes fainter, the jets 
feebler, the head larger, and the tail shorter, until the comet has 
reached its former low estate, having laid aside the gay trappings 
with which it was ornamented at perihelion. 

290. Jets and Envelopes. — The jets or fountains of matter which 

spurt out from the nucleus as the body nears perihelion are emitted 

from the sunward side of the nucleus, and are directed toward 

the sun. They rise higher and higher and 

become more diffuse, until they are lost in 

the head. One is exhibited in Fig. 150. 

A well behaved nucleus throws off en- 
velopes (Fig. 157). These rise sunward, one 
after another, becoming fainter and more 
diffused as they approach the outside of the 
head. 

291. Tails : their Dimensions and Varieties. 
— The tail is by far the bulkiest part of a 
comet. Tails long enough to reach from the 
earth to the sun are not uncommon. The extremity of such a 
tail is millions of miles in thickness. The tail of the comet of 
1843 was estimated to be 581,000000 miles long at one time. A 
tail a tenth as long as this is reckoned highly respectable. Some 
tails are narrow and straight, like prodigious spines. The forms of 
others are like half-opened fans. A comet occasionally has several 
tails, pointing in widely different directions. 

292. Companion Comets. — Comet 1889 V. (Brooks) was very re- 
markable on account of the group of companions which attended it. 
There were four of these comrades, moving along a little in advance 
of the main comet. 

Two of the companions were excessively faint, and finally disap- 




COMETS AND METEORS. I 93 

peared. The two brighter ones were perfect miniatures of the 
main comet, having tiny nuclei and shapely tails. But their 
beauty was evanescent. For a while they receded from the prin- 
cipal comet. In three weeks the nearer companion ceased to 
recede; it then enlarged, became very diffuse, and disappeared 
completely, as if blotted out of existence. The farther corn- 




Fig. 151. — Companions of Brooks's Comet. 

panion continued to recede, until it had become (a month after 
discovery) brighter than the main comet. In a month more it 
began to come back and shed its tail; its head swelled and became 
diffuse and faint, so that it appeared to be in a sorry plight. The 
companions may have been caused by a disruption of the parent 
mass by the attraction of Jupiter in 1886. At that time the comet 
was within the system of Jupiter's satellites for over two days and 
a half, and may have been struck by one of them ; it may even have 



194 DESCRIPTIVE ASTRONOMY. 

grazed the planet's surface. Companion comets are not very 
common. 

293. Constitution of the Head and of the Nucleus. — The most 
plausible theory is that the head is composed of a mixture of solid 
and gaseous matter. The presence of gas is shown by the spec- 
trum. The connection known to exist between certain comets and 
meteors (§ 332) renders it wellnigh certain that solid bodies are 
scattered throughout the head. That portions of the solid matter 
become liquid temporarily, when a comet like that of 1882 dashes 
through the sun's corona, is almost inevitable. 

The size of the solid bodies is largely a matter of conjecture. 
Some think that they are like grains of sand; others liken them 
to paving-stones and brick-bats. The nucleus is supposed to be 
the densest portion of this swarm of meteoric bodies. The nuclei 
of some large comets may be small solid bodies of great density. 

294. Evolution of the Tail. — Comets' tails point away from the 
sun, except in a few rare and anomalous instances. The material 

carried up by the jets and envelopes seems 
to be repelled by the sun, and driven 
away from it, despite its gravitational pull. 
The nature of the repulsive force is un- 
known, but it is generally thought to be 
electrical. Many experiments described 
in works on physics show that electrical 
attractions and repulsions, acting upon 
Fig. 152. — development light bodies which have a large surface in 
comparison with their mass, frequently 
overcome the force of gravity. A body, on the other hand, which 
has a small surface, but considerable weight, obeys the force of 
gravity. So the lightest portions of a comet's head might be 
driven away by an electrical repulsion originating in the sun, 
while the heavier portions, being but slightly affected by this re- 
pulsion, obeyed the law of gravitation. There is evidence that 
the nucleus, as well as the sun, repels the finely divided matter. 
Photographs of Rordame's comet, taken in 1893, 1 showed that cer- 
tain condensations in its tail were receding from the nucleus at 
the rate of fifty miles a second. 

1 By Prof. W. J. Hussey, at the Lick Observatory. 




COMETS AND METEORS. 



195 



295. Types of Tails. — A Russian astronomer, Bredichin, has 
made an elaborate investigation of the forms of comets' tails, and 
has divided them into three types. 

Tails of Type I. are nearly straight, and point almost directly 
away from the sun. They are thought to be composed of hydrogen. 



SUNWARD LINE 




Fi g- 153- — Type I. 

Type II. consists of the gently curving trains which are most 
common. These trains are probably composed of compounds of 
hydrogen and carbon, known to chemists as hydro-carbons : marsh 
gas and defiant gas are two of these. 



SUNWARD LINE 





SUNWARD LINE 



Fig. 154. — Type II. 



Fig. t 55. — Type III. 



Type III. is not common; it includes short tails of great curva- 
ture. The axes of such tails are far from pointing away from the 
sun. Hence they are composed of heavy materials, such as iron 
vapor. 

296. Mass and Density. — The masses of comets must be very 
small compared with those of the major planets; otherwise the 
earth and other planets would have been disturbed appreciably by 
comets which have come near to them. No such perturbation has 
ever been manifest. The combined mass of 100,000 bodies, each 
as massive as one of the greatest comets, would not equal that of 
the earth. Since they are of prodigious size, their mean density 
must be exceedingly small. Sir John Herschel watched a comet 
as it passed in front of a cluster of very minute stars; the lustre of 
the stars was not perceptibly dimmed. Even when a star is 
nearly behind the nucleus of a large comet, its light is scarcely 
dimmed. But careful observations of stars which were shining 



I96 DESCRIPTIVE ASTRONOMY. 

through hundreds of thousands of miles of cometary matter have 
shown that their light was refracted a trifle by the gases of the 
comet. While the mean density of a comet is low, the density of 
the particles of solid matter which make up most of its mass is 
probably comparable with that of the materials composing the 
earth's crust. Were the particles closely packed, the mean density 
would be largely increased. The tail of a comet is so tenuous that 
it may be appropriately likened to "such stuff as dreams are 
made on." 

297. Light and Spectra. — The brightness of a comet generally 
varies according to the changes of its distance from the earth and 
the sun. But there are frequent anomalous variations of brilliancy 
which would not take place were the light merely reflected sun- 
light. The spectrum of a comet is ordinarily a combination of two 
spectra: one is a faint continuous spectrum due to reflected sun- 
light ; the other is a spectrum of bright bands, like that produced 
by the flame of a Bunsen burner. It is due to glowing hydro- 
carbon gases. Comets which go near the sun, so as to be especially 
excited by its influence, exhibit spectra in which the lines due to 
sodium and iron are plainly visible. 

There is a rapidly accumulating weight of evidence in favor of 
the theory that a comet derives its light, except that portion which 
is merely reflected sunlight, almost wholly from electrical dis- 
charges between its particles. The electrical action is stimulated 
by the sun, being more intense the nearer the comet is to it. 

298. Fate of Comets. — The tail of a comet is not to be regarded 
as the same to-day that it was yesterday. When one of those 
comets which dash through the corona goes from one side of the 
sun to the opposite side in a few hours, the tail, though millions of 
miles in length, appears also to be swung around, like a gigantic 
scimetar brandished athwart the sky. No known force could cause 
the tail to swing around with such prodigious velocity. We there- 
fore conclude that the tail resembles the cloud of smoke puffed 
out from the smoke-stack of a locomotive. Fresh material is 
driven off from the head each second, to form the tail. As the 
smoke of a locomotive does not return to it again, so the particles 
driven off from the comet in such profusion at its perihelion pas- 
sage are lost in space. 



COMETS AND METEORS. 197 

Thus at each perihelion passage a periodic comet loses a portion 
of its mass. It must therefore in the course of ages be much 
reduced in mass and brightness. Sir Isaac Newton expressed the 
opinion that it was the fate of comets diffundi tandem et spargi 
percoelos universos, — "to be finally diffused and scattered through 
the celestial spaces." 

Comets which are not periodic, and which therefore do not visit 
the sun every few years, may in their infinite journeyings encounter 
other suns, swing about them in unwonted splendor, and continue 
their wanderings till they are captured for some sun by the aid of 
one of its planets, and are then gradually shorn of their beauty. 

299. Superstitions. 1 — Milton says, in the second book of Paradise 

Lost : — 

" On the other side, 
Incensed with indignation, Satan stood 
Unterrified; and like a comet burned, 
That fires the length bf Ophiuchus huge 
In the arctic sky, and from his horrid hair 
Shakes pestilence and war." 

Such superstitions have been rife from early times. Josephus 
mentions, among the prodigies which foretold the destruction of 
Jerusalem, a comet with a tail like the blade of a sword, which 
hung over the city a year. 

The Roman Emperor Vespasian, when nearing his end, heard 
some of his courtiers conversing in a low tone about a comet then 
visible. But he treated the matter lightly, saying, "This hairy 
star does not concern me; it menaces rather the king of the 
Parthians, for he is hairy and I am bald." 

Throughout the Middle Ages comets seem to have been regarded 
as especially presaging the death of kings. 

The comet afterward known as Halley's appeared in April, 
1066, when William the Conqueror was about to invade England. 
"Nova Stella, novus rex," was the proverb of the time. A monk, 
apostrophizing the comet, said : " Here art thou, source of the 
tears of many mothers. Long have I seen thee; but now thou 
appearest to me more terrible, for thou menacest my country with 
complete ruin." 

1 See the English edition of Guillemin's " World of Comets." 



198 



DESCRIPTIVE ASTRONOMY. 



The comet of 1528 must have struck terror to the hearts of the 
beholders. Ambrose Pare, one of the most learned men of that 
time, writes of it as follows: — 

'• This comet was so horrible, so frightful, and it produced such great 
terror in the vulgar, that some died of fear and others fell sick. It appeared 
to be of excessive length and was of the color of blood. At the summit of 




Fig. 156. — Comet of 1528. 



it was seen the figure of a bent arm, holding in its hand a great sword, as if 
about to strike. At the end of the point there were three stars. On both 
sides of the rays of this comet were seen a great number of axes, knives, 
blood-colored swords, among which were a great number of hideous human 
faces, with beards and bristling hair." 



COMETS AND METEORS. 1 99 

Through the instrumentality of modern science the terror 
formerly inspired by great comets has largely given place to a 
lively delight in watching their beautiful forms and wonderful 
changes. 

300. Collisions. — Comets are still feared by many people, on the 
ground that they may collide with the earth and arrest its motion, 
so that it will begin to fall toward the sun, or that they may 
produce such intense heat by the impact that, in the words of 
Prospero, — 

" The great globe itself, 
Yea, all which it inherit, shall dissolve, 
And, like this unsubstantial pageant faded, 
Leave not a rack behind." 

From what we have already learned concerning the masses of 
comets, it is plain that there is no ground for apprehension that 
the earth's orbit would be much changed by collision with even 
the largest of them. Computations of the orbit of the magnificent 
comet of 1 86 1 (Fig. 157) showed that the earth probably passed 
through its tail on a certain night. The result was no more 
serious than if our planet had been smitten by the club of a 
phantom. If the earth encountered the head of an ordinary 
comet, the meteoric masses of which it is presumably composed 
might be dissipated into vapor when they struck the atmosphere. 

Should these particles prove to be metallic masses as large as 
the fist, able to plough through the atmosphere, and to make a 
fiery rain upon the earth's surface, the bombardment would be 
memorable. 

While there are no very definite data to reason from, it is be- 
lieved that an encounter with the nucleus of one of the largest 
comets is not to be desired. 

So vast are the celestial spaces in comparison with the bodies by 
which they are peopled, that the chance that the earth will strike 
a good-sized comet some time during the next 100,000 years is 
exceedingly slight. 

Most astronomers would be delighted with the prospect that the 
earth was going to blaze a pathway through some ordinary comet. 
They would also be pleased to watch some large comet dashing 



200 



DESCRIPTIVE ASTRONOMY. 



headlong into the sun. Professor Young thinks that the heat 
evolved by the collision would be chiefly liberated below the 




Fig. 157. — Comet of 1861. 

solar surface, and would simply add a trifle to the sun's store of 
energy. 

In November, 1892, when it was supposed that Biela's comet 
was about to strike the earth, there was considerable fright. The 



COMETS AND METEORS. 201 

following despatch from Atlanta, Georgia, was printed in a daily 
paper. 

" The fear which took possession of many citizens has not yet abated. 
The general expectation hereabouts was that the comet would be heard 
from on Saturday night. As one result, the confessionals of the two 
Catholic churches here were crowded yesterday evening. As the night 
advanced there were many who insisted that they could detect a change 
in the atmosphere. The air, they said, was stifling. It was wonderful to 
see how many persons gathered from different sections of the city around 
the newspaper offices with substantially the same statement. As a conse- 
quence, many families of the better class kept watch all night, in order that 
if the worst came they might be awake to meet it. The orgies around the 
colored churches would be laughable, were it not for the seriousness with 
which the worshippers take the matter. To-night (Saturday) they are all 
full, and sermons suited to the terrible occasion are being delivered." 

REMARKABLE COMETS. 

301. Halley's Comet. — Halley, who was a contemporary of New- 
ton, having learned that, according to the recently propounded 
theory of gravitation, comets might move in elliptic orbits, and 
thus be visible at several returns, computed the orbits of two dozen 
comets. On comparing the computations he observed that the 
orbits of the comets of 1531, 1607, and 1682 were strikingly simi- 
lar; he reasoned from this that these three were one and the same 
body, revolving about the sun in about seventy-five years. He 
predicted its return about 1758; knowing that he would be in his 
grave before that time, he expressed a modest hope that, if the 
comet should return then, "posterity would not refuse to acknowl- 
edge that this was discovered by an Englishman." In 1757 
astronomers began to watch for it. For weary months the quest 
was vain. Clairaut, a French mathematician, showed by an elab- 
orate investigation, which challenged the admiration of the world, 
that the comet would be retarded 618 days by the attraction of 
Jupiter and Saturn, and would arrive at perihelion in the middle 
of April, 1759. He said that this might be a month in error. The 
comet was first seen on Christmas night, 1758, and arrived at 
perihelion on March 13, a month before the time set. Thus the 
Newtonian principle of gravitation received a striking verification. 



202 DESCRIPTIVE ASTRONOMY. 

This comet was seen many times before Halley's day. Its 
earliest recorded appearance is supposed to be that of B. C. u. It 
is expected again in 1910 or 191 1. 

302. Encke's Comet. — This comet is insignificant in appearance, 
rarely exhibiting a sharply defined nucleus, and sporting the scan- 
tiest of tails. It was discovered in 1786, and thirty-two years after- 
wards was found to be revolving in a period of only three years 
and a quarter, the shortest known period. It is specially interest- 
ing because its motion suffers much at the hands of Mercury, and 
is also accelerated in a strange way. Encke's laborious computa- 
tions showed that its periodic time was shortened nearly three hours 
at each revolution. More recent observations and discussions show 
a reduction of the acceleration to one half its former value. The 
acceleration was at one time considered a triumphant proof of the 
existence in space of a resisting medium, the luminiferous ether. 
Such a medium, by retarding the comet's motion, would cause it 
to drop toward the sun, and revolve in a smaller orbit, in which it 
would move with greater speed. But the recent diminution of the 
acceleration does not bear out this theory. The comet's disturbance 
may be due to collisions with small bodies coming across its path, 
or to its own feeble activities in the line of throwing off envelopes 
or jets. 

Its perturbations by Mercury afford a means of determining the 
mass of that planet. 

303. Biela's Comet. — This was discovered by Biela, an Austrian, 
in 1826; its periodic time was computed to be 6V± years. It was 
due again in 1832, and then gave rise to the first comet scare of 
this century. For calculation showed that it came close to the 
earth's orbit, and ignorant people became much alarmed at the 
prospect of a collision. Though it came near the earth's orbit, 
the earth itself was at no time nearer to it than 15,000000 miles. 
In December, 1845, it was found to be elongated, and ten days 
afterward it split into two comets, one of them being much smaller 
than the other. Both became brighter and developed well defined 
nuclei and tails. The smaller one grew in brightness until it 
surpassed the other for a time. Their distance apart became 
157,000 miles, and they were lost to view in April, 1846. 

In 1852 the twins were seen again; the distance between them 



COMETS AND METEORS. 203 

had increased to a million and a half miles; sometimes one was the 
brighter, sometimes the other. They faded from sight again, and 
have never been seen since, though they were in a very favorable 
position in 1872. Comet Holmes, which appeared in November, 
1892, was at first thought to be the long lost Biela, but the compu- 
tation of its orbit showed that this was not the case. 

304. The Great Comet of 1882. — This was discovered early in 
September by observers in the southern hemisphere. On Septem- 
ber 17, the observers at the Cape of Good Hope saw this aston- 
ishingly brilliant body move swiftly up to the limb of the sun, 




Fig. 158. — The Great Comet of 1882. 

and then vanish completely as it swept over its broad disk. On 
the following day it was seen close to the sun by observers all over 
the world. It was only necessary to screen off the sun's light by 
holding up the hand at arm's length in order to see the comet. 
In less than a week the nucleus, formerly round, became oval, and 
by the end of a month two centres of condensation were seen. 
During the next three months the nucleus became divided into 
four or five condensations, ranged in a line and connected by a hazy 
mass of light. 

Meanwhile a magnificent train, 100,000000 miles in length, had 
been developed. More than half a dozen companion comets were 



204 



DESCRIPTIVE ASTRONOMY. 



seen, all evanescent. The fiery object seemed to be strewing its 
path with filmy debris, thrown off by some unknown force. Possi- 
bly they were fragments driven off by the intense repulsive action 
of the sun, as the comet dashed through the corona. When at 
perihelion, it was less than 300,000 miles from the sun's surface. 

There was a faint but prodigious sheath of cometary matter 
which enveloped most of the comet, and projected millions of miles 






Fig. 159. — Nucleus of the Great Comet of 1882, at Different Times. 

in front of the head. The orbit was not appreciably changed by 
any resistance due to the coronal matter. The periodic time is 772 
years. 

Besides the hydro-carbons ordinarily found in comets, there were 
in this body sodium and iron; some of the numerous bright lines 
in the spectrum were probably due to calcium and manganese. 

305. Swift's x Bright Comet of 1892. — This is mentioned on ac- 
count of the marvellous changes observed in its tail. On April 4 
it was 20 long, straight and slender; in the telescope it was seen 
to consist of two branches, between which scarcely any cometary 
matter was visible. The next morning a new tail had formed 
between the other two, and each tail was composed of several 
lying close together. At least a dozen could be counted. After 
the lapse of another day, one of the original three tails had van- 
ished, and the other two were blended. 

Then one of these grew bright, and the other faded away; the 
bright one had a sharp bend in it, as if turned aside by some 



1 Lewis Swift, of Echo Mountain, California. 



COMETS AND METEORS. 



205 





Fi?. 160. — Swift's Comet: Photographed by Barnard. 



206 DESCRIPTIVE ASTRONOMY. 

obstacle. Near the point of deflection were two dark spots in the 
brightest part of the tail. Finally the tail split up into six 
branches. All these changes and some others took place in five 
days. 

306. Comet Holmes. — This was discovered on Nov. 6, 1892, by 
Mr. Holmes, an English amateur astronomer. Its position in the 
sky was near that in which Biela's comet might appear, and the 
latter (if still in existence) was due. Therefore the comet was 
supposed by some to be Biela's, and preliminary computations led 
to the interesting result that the comet was likely to collide with the 
earth. On this account a wide-spread popular interest was awak- 
ened (§ 303). The comet changed its position in the sky so little, 
for several weeks, that its orbit could not be computed with much 
accuracy at first. It was finally found to be moving in a small 
ellipse, which is more nearly circular than the orbit of any other 
known comet; the period is less than seven years. The orbit 
resembles that of an asteroid. The comet, which was at first vis- 
ible to the naked eye, grew faint and diffuse in a few weeks. But 
in the middle of January, 1893, it suffered a strange transformation, 
changing into a starlike object surrounded by a small circular 
nebulosity. On January 16, Dr. Barnard saw the nucleus brighten 
considerably while he was observing it. During the next week the 
nucleus suffered marked changes of brightness, being very plain at 
times, and almost invisible at others. The surrounding nebulosity 
soon grew larger and fainter, and faded away. 

307. Comet c 1893 (Brooks). — This comet was discovered on the 
morning of Oct. 17, 1893, by William R, Brooks, of Geneva, N.Y. 
At first it had a short tail issuing from the northern side of the 
head, in addition to the main tail, which was straight and of a 
graceful form. But a photograph taken by Dr. E. E. Barnard on 
the morning of October 21 (October 20 by astronomical time) 
showed remarkable changes in the tail, which Dr. Barnard thus 
describes. 1 

" It presented the comet's tail as no comet's tail was ever seen before. 
The graceful symmetry was destroyed ; the tail was shattered. It was 
bent, distorted, and deflected, while the larger part of it was broken up 

1 Popular Astronomy, December, 1893. 



COMETS AND METEORS. 



207 



into knots and masses of nebulosity, the whole appearance giving the 
idea of a torch nickering and streaming irregularly in the wind. The 
short northern tail was swept entirely away, and the comet itself was 
much brighter. 

" The very appearance at once suggested an explanation, which is prob- 
ably the true one. If the comet's tail, in its flight through space, had 
suddenly encountered a resisting medium which had passed through the 
tail near the middle, we should have precisely the appearance presented 
by the comet. It is not necessary that the medium should be a solid 
body ; if it possessed only the feeblest of ethereal lightness it would de- 
flect, distort, and shatter the tail. What makes this explanation all the 
more probable is that the disturbance was produced from the side of 
the tail that was advancing through space." 




Fig. 161. — Brooks's Comet: Photographed by Barnard. 

The appearance may also be explained by variations in the 
amount and in the direction of motion of matter driven off from 
the comet's head. 

METEORS. 



308. The Two Classes. — Meteors are divided into two classes, 
meteorites and shooting stars. Meteorites are the bright bodies 
which from time to time dash through the air, like balls of fire, 
and fall to the ground. There are various other names for them, 
the most common one being aerolites. Brilliant meteoric objects 
which do not fall to the earth are ordinarily designated only bv the 
general term "meteor." Shooting stars, on the other hand, are 
less conspicuous bodies, which can be seen on any clear dark 



208 DESCRIPTIVE ASTRONOMY. 

night, darting across the sky; they usually attract no especial 
attention by their brightness, and never fall to the earth. 

309. Past Appearances of Meteorites. — Falls of meteorites are 
recorded before the present era. Though there are many records 
of the falling of stones from the sky, they were at the close of the 
last century regarded by most scientific men as old wives' fables. 

There is one at Mecca, which is adored by the faithful Mussul- 
man. An Emperor in the Middle Ages was said to have a sword 
which was fashioned from one of these celestial visitors. In April, 
1889, copper earrings plated with meteoric iron were found in an 
Indian mound in Ohio. During the last decade of the eighteenth 
century, several falls of meteorites occurred, which were so reli- 
ably substantiated that scientific men began to inquire into the 
matter earnestly. 

In 1803 such a shower fell at L'Aigle in Normandy, that the 
French Academy sent one of their number to inquire into the 
matter. So exhaustive was his investigation, and so convincing 
his report, that the most sceptical were forced to admit that stones 
fell from the sky. The story of one of the best authenticated as 
well as most remarkable of meteorites is told in the next section. 

310. The Ensisheim Meteorite. — This meteorite fell on Nov. 7, 
1492, at Ensisheim, in Alsace, and an account of it, together with 
the stone itself, was put in the church at that place. The account 
states that on the day in question, some minutes before noon, there 
was a loud noise, like the rumbling of thunder, and a stone weighing 
280 lbs. was seen by a child to dash into a ploughed field and bury 
itself about three feet in the earth. Some small pieces of it were 
taken for examination, but the parent mass was suspended in the 
choir of the church. There it remained until it was ruthlessly taken 
away during the French Revolution. It has since been restored 
to the town hall of the village. 

311. A Detonating Meteor. — On Dec. 21, 1876, a superb fireball 
appeared over the State of Kansas, and moved thence eastward south 
of Chicago across Indiana, over Lake Erie, to Lake Ontario, where 
it disappeared. When nearly 200 miles from Bloomington, Indiana, 
the meteor burst, and the inhabitants of that city saw a magnificent 
array of fireballs sweeping through the evening sky. After the ex- 
citement aroused by the marvellous spectacle was over, there came a 



COMETS AND METEORS. 209 

tremendous crash, like the heaviest reverberations of thunder. The 
concussion which accompanied it led some to think that a light 
earthquake had shaken the town. How terrific must a detonation 
have been, which was so startling nearly 200 miles away, after the 
sound waves had been on their journey a quarter of an hour ! 

312. Kiowa County (Kansas) Meteorites. — In March, 1890, the 
attention of scientific men was called to several strange pieces of 
iron, which had been ploughed up from time to time in Brenham 
township, Kiowa County, Kansas. They had been put to various 
uses, such as holding down the cover of a rain barrel, and keeping 
the roof of a stable from blowing away, and helping to stop a fence 
hole through which hogs escaped from their feeding ground. One 
had risen to the dignity of ornamenting the sidewalk in front of a 
real estate office. 

A cowboy and a woman were the only people in the vicinity who 
seemed to realize the value of these articles. The cowboy, being 
unable to carry his off, buried them, and died shortly afterward. The 
woman sent for a college professor, and disposed of hers for enough 
to pay off the mortgage on her farm. 

The meteorites were found scattered over an oval area about a 
mile long. The largest mass, called by the farmers the " moon me- 
teorite," weighed 466 pounds. The total weight of the twenty odd 
pieces found was about a ton. 

313. A Meteorite in Flight. — A flying meteorite is an object of 
dazzling splendor, when seen by night. It is generally followed by 
a luminous train, which may remain for some minutes. A noise like 
the roar of artillery is heard, with occasional crashes like those 
caused by the explosion of shells, which signalize the breaking of 
the main body. 

When the meteorite is many miles away, one may see a flash of 
light without hearing the accompanying detonation. In a very few 
seconds all is over, save that there is a bright irregular streak of 
light on the sky, marking the meteor's path. A recent meteorite is 
reported as having the distinct shape of a banana, and as turning end 
for end as it flew, scattering sparks along its pathway: it glowed 
like a piece of red-hot iron, though the sun was half an hour above 
the horizon. 

The meteor's velocity is so effectually checked by the resistance 



2IO 



DESCRIPTIVE ASTRONOMY. 



of the air, that it finally comes to earth like a spent cannon ball or 
a shower of grape. 

314. Path and Velocity. — Meteorites move in orbits about the 
sun : ninety per cent of those orbits which have been computed are 
elliptical. These bodies generally become inflamed at a height of 
eighty miles or more above the earth's surface, despite the rarity of 
the air at that elevation. The path is occasionally hundreds of miles 




Fig. 162. — Meteor seen at Bassein, Burmah. 
(From Winchell's " World Life," by permission.) 



in length. The velocity of a meteorite with reference to the earth 
depends upon the relative directions of their motions. When the 
collision occurs "head on" the relative velocity exceeds forty miles 
a second. When the meteor catches up with the earth, both being 
in motion in the same direction, the relative velocity sinks below ten 
miles a second. The velocity with which the meteor reaches the 
ground is often considerably less than that of a cannon ball, as is 
shown by the slight depth to which it penetrates. Notwithstanding 
this comparatively low velocity, a meteorite is no insignificant mis- 
sile. There are a few records of the destruction of buildings by 
them. The Chinese, not to be outdone by Western nations, have a 
record of one which came to earth 2,500 years ago, destroying 
several chariots and killing ten men. 



COMETS AND METEORS. 211 

Though the meteorite which flew into fragments over Madrid 
on Feb. 10, 1896, was about fifteen miles above the city, the con- 
cussion caused strong vibrations of partitions of houses, and exten- 
sive damage to windows. 

315. Cause of Light and Heat. — When the motion of a cannon ball 
is arrested by striking an armor plate, the ball and the plate are 
heated, so that the armor plate becomes viscous at the point of 
striking, and flows like tar. The energy possessed by the ball be- 
cause of the swiftness of its motion is transformed into heat when 
the motion is arrested. By the same principle a nail is heated, 
when struck repeatedly by a hammer. The energy in the mov- 
ing hammer is changed into heat. As the speed of a rifle ball is 
checked when it is fired into water, the speed of a meteorite, 
which may be one hundred times as great as that of the rifle ball, 
suffers disastrous diminution even in the upper regions of the 
atmosphere, and is almost destroyed before the body reaches the 
ground. The energy lost by the meteor, as its speed diminishes, 
reappears in heat. The air is heated enormously ; the quantity of 
heat developed, if it were all spent on the meteorite, would liquefy 
it, were it of iron. The meteor shines, because its surface is 
intensely hot: most of the light which we see undoubtedly comes 
from the incandescent gases surrounding the meteor. The train 
left behind remains luminous for so long a time that its light 
cannot be accounted for by heat alone : it may possibly be due 
to phosphorescence. 

316. Effect of the Heat. — If a candle were thrown through the 
flames of a large bonfire, some of its surface would be melted off, 
but the interior of the candle might not be heated perceptibly. In 
like manner, a meteor when dashing through the heated air is 
affected as if it were passing through a sea of burning gas raging 
with uncontrollable fury. The outside of the meteor is fused at 
once, and wiped off to form the train. As the exterior is very hot, 
the meteorite is liable to crack, and strew its path with its own 
debris. Sometimes the heating causes a terrific explosion, or a 
series of explosions, which reduce the meteor to small fragments. 
So rapid is the entire process that the heat at times does not 
penetrate to the interior of the mass. While most pieces of freshly 
fallen meteorites are too hot to handle, some are cold. A portion 



212 DESCRIPTIVE ASTRONOMY. 

of one which fell in India was found, about half an hour afterward, 
embedded in moist earth, and coated with ice. 

The intensity of the heat to which a meteor is exposed may be 
illustrated by the case of a fireball which was observed in England 
in 1869. The luminous fiery envelope was more than four miles in 
diameter, and the entire meteor was vaporized in five seconds, while 
travelling 170 miles. There remained a cloud of glowing vapor 
about fifty miles long and four miles broad, which was visible for 
fifty minutes. 

317. Meteoric Stones. — Most of the meteoric masses which fall to 
the earth are of a stony nature. When found they are glazed over 
with a thin crust formed by the fusion of the exterior during the 
flight. When a meteorite bursts just before falling, the freshly 
broken surfaces are not thus incrusted, and the pieces have in some 
cases been fitted together again. The surface is usually indented 
with numerous pits caused by the fusion of parts of the meteoric 
mass. The structure of some of these objects is like that of certain 
volcanic rocks, which are formed of irregular masses of various ma- 
terials held together by a cement. In half a dozen meteorites com- 
pounds of carbon have been found, which are like those resulting 
from the decay of vegetable life ; but no forms of vegetation such as 
we frequently find in terrestrial sandstones have been discovered. 

318. Meteoric Iron : Intermediate Forms. — A small percentage of 
meteorites are composed of iron, which is alloyed with nickel in all 

yet analyzed. Only about a 
dozen of these have actually 
been seen to fall. The others 
have been found lying on or 
near the surface, in places 
where there is no other iron in 
the vicinity. A few of these 
masses weighed several tons. 

There are forms intermedi- 
ate between meteoric iron and 
Fig. 163.- ^ Meteorite. stone j n some of these there 

(From "Science," by permission.) . 

is a honeycombed mass of iron, 
the cavities in which are filled with various minerals. In others bits 
of iron are found scattered through a stony mass. 




COMETS AND METEORS. 



213 



319. Elements found in Meteorites. — More than a third of the 
known chemical elements are found in meteorites ; no new element 
has been discovered in them, but some new compounds have been 
found. Most of the elements are common ones, such as sulphur, 
phosphorus, copper, tin, aluminum, calcium, etc. There are, as 




Fig. 164. — The Canyon Diablo Meteorite. 



yet, no traces of gold or silver. Small black diamonds, and also 
minute white ones, have been found in cavities of meteoric iron, 
which came from the Canyon Diablo, in Arizona, where about 
three hundred fragments of meteoric iron were discovered in 1891 : 
the largest piece, weighing 1,015 pounds, was on exhibition at the 
World's Fair in Chicago. Dust from the diamonds was employed 
in the Tiffany pavilion at the Fair, to polish other diamonds : the 
success of the experiment proved conclusively that the hard black 
grains from the meteorite were genuine diamonds. 

320. Origin. — The Austrian mineralogist, Tschermak, after a care- 
ful study of the structure of meteorites, reached the conclusion that 
they had a volcanic origin. The volcanoes of the moon are now ex- 
tinct. No terrestrial one has sufficient power to eject a missile with 



2X4 DESCRIPTIVE ASTRONOMY. 

a velocity of six miles a second, which would be necessary to carry 
it away from the earth's attraction, if the earth had no atmosphere, 
and to render it obedient to the sun. But when these bodies or 
other planets were young, their volcanoes may have possessed the 
necessary energy. The sun, as we have mentioned (§ 77), has been 
seen to eject masses of heated vapor with such a velocity that they 
would not fall back again. Such masses, exposed to the cold of 
space, would be condensed into solid bodies, like the meteorites. 
It has been the general belief among astronomers that meteorites 
originated like the large heavenly bodies, from the condensation of 
nebulous matter scattered throughout the universe. But Tscher- 
mak's theory is considered worthy of careful examination. 

321. How to Observe a Meteorite. — The observations necessary for 
determining a meteor's path can be easily made by any intelligent 
person who gets a fair view of it. Two things are to be observed, 
the time and the direction of movement. 

At night one familiar with the constellations may note its path 
among the stars, and the time at which it disappears. By repeating 
" Mary had a little lamb," etc., beginning when the meteor is first 
seen and stopping at its disappearance, the length of time during 
which it is visible can be estimated ; for the number of seconds 
required to repeat the same snatch of rhyme is easily obtained 
afterwards by rehearsing it, watch in hand. In the daytime, the 
position of the body when first and last seen can be noted with refer- 
ence to surrounding objects. The altitude and azimuth (§ 12 1) of 
the object at each of these times can be measured later with a sur- 
veyor's transit, placed at the point where the observer stood. 1 

SHOOTING STARS. 

322. Numbers. — One can scarcely look at the face of the sky for 

fifteen minutes on a clear moonless night without seeing at least one 
of these objects dart harmlessly across the sky, and disappear in 
silence, leaving no trace behind. It has been estimated that if ob- 

1 A careful report of such observations, however rude they may seem to the ob- 
server, would be welcomed by any astronomical periodical. Such communications may 
be sent to the " Astronomical Journal," Cambridge, Mass.. or to '•' Popular Astronomy," 

Northfield, Minn. 



COMETS AND METEORS. 215 

servers were uniformly distributed over the earth, in such a way as 
to command a view of all the shooting stars entering the atmosphere, 
ten or fifteen million of them would be found to strike the earth 
during a day. 

They are twice as frequent at 6 A. M. as at 6 P. M. For in the 
morning we are in front of the earth, as it moves in its orbit, while in 
the evening we are in the rear, as shown in Fig. 165. At A the sun 
is rising : at B it is setting. The meteors are supposed to be com- 
ing from all directions. 



SUN 




323. Paths and Velocity. — A shooting star which is coming di- 
rectly toward the observer has no visible path. It is simply an 
evanescent bright spot on the sky. Those which shoot to one side 
of him usually have paths several degrees in length. The paths of 
some meteors exhibit abrupt changes of curvature : the meteors 
appear to glance, as a skipping stone does on the water. 

By means of observations taken by men stationed several miles 
from each other, shooting stars have been found to be on the aver- 
age seventy-five miles above us when they are ignited, and fifty 
miles when they disappear. While glowing they travel forty or 
fifty miles at rates of from ten to fifty miles per second. Like the 
meteorites, they have orbits about the sun. A few double meteors 
have been seen moving side by side, some of them being connected 
by a ligament of light. They were telescopic. 

324. Masses and Constituents. — Since these bodies perish when 
they have encountered the extremely rare atmosphere which exists 
at heights of from fifty to seventy-five miles, they must be insignifi- 



2l6 



DESCRIPTIVE ASTRONOMY. 



cant in mass. Most of those which compose a shower are believed 
to be less than a grain in weight. One as brilliant as Jupiter or 
Venus at their best, may weigh 50 or 100 grains. These rather 
insecure estimates are based upon measurements of their light, 
combined with those of their velocity. The spectroscope shows 
that sodium and magnesium are constituents of shooting stars. 

Some years since, the Swedish naturalist Nordenskiold melted a 
quantity of snow taken from polar regions, and discovered in the 
water minute particles which proved to be compounds of iron : they 
were hailed as the debris of shooting stars. 

But the great eruption of Krakatoa in 1883 taught us that fine 
volcanic ashes may be carried in the air for thousands of miles be- 
fore they finally settle to the earth. The particles found by Nor- 
denskiold may therefore be the products of volcanic eruptions. 




Fig. 166. — The Radiant. 



325. Radiant. — When a shower of shooting stars comes, the ap- 
parent paths of the bodies when produced backward meet at a spot 
on the sky, which is called the radiant point, or simply the radiant. 
One who looks upward during a gentle fall of snow will observe the 
same phenomenon with reference to the paths of the snowflakes. 

The paths prolonged backward seem to converge toward the 
zenith. But the snowflakes are descending in parallel paths, and 
the convergence is explained as an effect of perspective. Hence 



COMETS AND METEORS. 21 7 

we infer that the shooting stars are coming in parallel paths. As the 
hours of the night wear away, the radiant remains in the same place 
among the stars : from this we conclude that the meteors come from 
the same direction, so that there must be a stream of them, pouring 
a bootless fusillade in upon the earth. 

Meteoric showers are frequently named from the constellation in 
which the radiant lies. The Leonids, Perseids, and Andromedes 
come from the constellations Leo, Perseus, and Andromeda, re- 
spectively. 

326. The August Shower. — The August meteors are popularly 
known as the " Tears of St. Lawrence." They are most numerous 
about August 10, when an observer may see one every minute, 
the radiant being in the constellation Perseus. Perseids are visible 
in greater or less numbers during the latter half of July and the first 
three weeks of August (Fig. 167). This shows that the meteoric 
stream is very broad, for the earth moves about 60,000000 miles 
while passing through it. 

The meteors are distributed rather uniformly along the orbit, 
though there are occasional gaps. In August, 1892, the shower did 
not come. The orbit is an ellipse extending beyond the orbit of 
Neptune, and the period of revolution is over 100 years. 

327. The November Leonids: Appearance: Velocity: Orbit. — Every 
year about November 13 there is a shower of meteors emanating 
from the constellation Leo. In most years they are rather insig- 
nificant, but once in 33 years the magnificence of the display is 
appalling. When the encounter takes place, the meteors come in 
a direction nearly opposite to that in which the earth is moving. 
The velocity of the meteors is 26 miles a second, and that of the 
earth 18 miles a second, so that the missiles pelt the earth as furi- 
ously as if they were going 44 miles a second, and the earth were 
at rest. They have a brilliant bluish light, and leave vivid trails. 

This meteoric system is not diffused around its orbit, like the 
August meteors, but is largely condensed in a single shoal about 
2,000,000000 miles long. Their orbit is a long ellipse, the peri- 
helion of which is near the path of Uranus. The periodic time is 
33I years. 

328. The November Leonids : Past Showers. — The earliest re- 
corded appearance of this shower was in 902 a. D. On the very 



2l8 



DESCRIPTIVE ASTRONOMY. 




Fig. 167. — The Orbit of the August Shower. 
(From Winchell's " World Life,' 1 by permission.) 



night when a Moorish tyrant died, " by the judgment of God," there 
were seen, " as it were lances, an infinite number of stars, which 
scattered themselves like rain to right and left." That year was 



COMETS AND METEORS. 2 I 9 

called the "Year of the Stars." On the night of Nov. 12, 1833, the 
display was probably the most magnificent on record. The falling 
stars were as thick as flakes in a snowstorm ; there were many fire- 
balls brighter than Venus ; one is said to have looked larger than 
the full moon. The utmost terror was inspired in the ignorant. A 
South Carolina planter wrote : — 

" I was suddenly awakened by the most distressing cries that ever fell 
on my ears. Shrieks of horror and cries for mercy I could hear from most 
of the negroes of the three plantations, amounting in all to about six or 
eight hundred. While earnestly listening for the cause I heard a faint 
voice near the door calling my name. I arose, and, taking my sword, stood 
at the door. At this moment I heard the same voice still beseeching me 
to arise, and saying, ' The world is on fire ! ' I then opened the door, and 
it is difficult to say which excited me the most, — the awfulness of the 
scene, or the distressed cries of the negroes. Upwards of one hundred lay 
prostrate on the ground, — some speechless, and some with the bitterest 
cries, with their hands raised, imploring God to save the world and them. 
The scene was truly awful ; for never did rain fall much thicker than the 
meteors fell to the earth : east, west, north, and south, it was the same." 

In 1866 there was another wonderful display, on the night of 
November 13. The next great shower is expected in 1899; the 
meteoric shoal is so long, that there may be a fair shower in 1898. 

329. History of the Leonids. — Fig. 168 illustrates the supposed 
introduction of the Leonids into our system. Their probable his- 
tory is as follows. Prior to the second century of our era they were 
coming toward the solar system in a tolerably compact swarm mov- 
ing in a parabolic orbit. Neptune, the first picket, was successfully 
passed. But Uranus came along opportunely, and in A. D. 126 gave 
them so powerful a tug that their orbit was changed to the ellipse 
shown in the figure, and they found themselves subject to the sun. 
The attraction of Uranus also distorted the shoal, and caused its 
various components to move in slightly different orbits : the sepa- 
rate meteors were made to move with slightly different velocities, so 
that the shoal became elongated. Time and time again the earth 
dashed through it. Furthermore, the attractions of Jupiter and 
Saturn kept shifting its orbit, until it now occupies the position 
shown by the dotted lines in the figure. Looking ahead, we may 



220 



DESCRIPTIVE ASTRONOMY. 



prophesy that the shoal will, in the course of ages, become dis- 
tributed around the orbit, as are the August meteors. 

330. The Bielids. — This meteoric shower comes in the latter part 
of November each year. It overtakes the earth, striking it with a 




Fig. ii 



Capture of the Leonids. 



relative velocity of only 12 miles a second. The radiant point is in 
Andromeda. Brilliant displays were seen in 1872 and 1885. 1° 
1892 the meteors were expected on November 27, but arrived on 
November 23. The early arrival was afterwards discovered to be 
an effect of Jupiter's attraction upon the meteoric stream. Com- 
parison with old records shows that the shower, though somewhat 
irregular in the date of its appearance, comes gradually earlier and 
earlier, gaining a fortnight in a century. The swarm derives great 
interest from its supposed connection with Biela's comet (§ 303). 
The comet seems to be lost, but a meteoric shower more pronounced 
than the average comes when the comet is due. Some writers speak 
of these meteors as fragments of Biela's comet. In the shower of 
1892 several could be seen every minute by a single observer. 
Another fine display is expected in 



COMETS AND METEORS. 221 

331. Meteoric and Cometic Orbits. — As soon as the orbits of the 
great meteoric swarms were computed, it was perceived that they 
were similar to those of certain comets. The August meteor swarm 
moves in an orbit which is identical with that of the bright comet 
of 1862. 

The meteors which cause the great 33 year November shower 
follow hot on the trail of Tempel's comet (1866, I.). 
Several other similar coincidences are now known. 

332. Relation between Comets and Meteors. — The Bielids are con- 
sidered by many to be portions of Biela's comet. 

The bright comet of 1862 seems to be simply a condensed por- 
tion of the August meteoric swarm. 

Tempel's comet and the 33 year shower may both be parts of 
the original mass brought into our system by the attraction of 
Uranus. 

The general opinion is that shoals of meteoric matter ac- 
companying comets are the products of the disintegration of the 
cometary masses. 

333. How to Observe a Meteoric Shower. 1 — In order to calculate 
the orbit of a shower it is necessary to know the position of the 
radiant point. A careful map of the stars visible to the naked eye 
in the vicinity of the radiant is first prepared. This may be done 
easily by putting a piece of tracing paper or tracing cloth over a 
good star map. On tracing cloth the stars may be well marked by 
small dots of ink. The completed map should be securely fastened 
to a smooth board. A watch of the sky for a few minutes during 
the shower will enable one to locate the radiant point fairly. With 
eyes directed toward this spot, the observer notes the apparent path 
of some meteor ; he then traces it upon the map, marking as accu- 
rately as possible the beginning and end of the path. A stick held 
in the hand, and placed parallel to the meteor's path, will be of de- 
cided assistance. Those meteors having short paths, are best suited 
for fixing the position of the radiant. When the observations are 
finished, the paths marked on the map should be prolonged back- 
ward till they intersect : they will not all intersect at the same point. 
After studying the map, the observer should mark the spot which 

1 See articles by W. F. Denning in "Popular Astronomy " for October, November, 
and December, 1893. 



222 DESCRIPTIVE ASTRONOMY. 

he deems to be the true position of the radiant, and find its right 
ascension and declination. 

Another person, or several others, may make repeated counts of 
the number of meteors visible in ten minutes, noting the time when 
each set of observations was begun. 

Still other observers may note the paths of brilliant ones, and in- 
teresting phenomena concerning them, such as their brightness, 
color and length of train. Successful observations should be pub- 
lished as recommended in the note to § 321. 

The apparent paths of the brighter meteors can now be de- 
termined with more accuracy by photography, than by visual 
observations. 

THE ZODIACAL LIGHT. 

334. In the early spring one may see, when twilight has faded 
away, a faint hazy beam of light projecting up from the horizon in 
the west. It lies in the ecliptic, and can be traced 90 or more from 
the sun without much difficulty. 

In autumn it can be well seen in the morning before sunrise. It 
is said to have been observed to follow the ecliptic clear around the 
sky. There are many theories as to its cause : the most widely ac- 
cepted is that it is due to a countless host of meteoric bodies revolv- 
ing about the sun, and constituting a huge figure resembling in 
shape a double convex lens. 

The Gegenschein, or Zodiacal Counterglow, lies opposite the 
sun, and is a very faint round appearance, a trifle brighter than the 
adjoining portions of the zodiacal light. Observations of it are 
exceedingly difficult. 1 

EXERCISES. 

335. 1. May Comet a 1907 be also Comet 1907 II.? 

2. Why are jets spurted out from the nucleus of a comet on the 
side next to the sun rather than on the other side ? 

3. Why are comets' tails when composed of a heavy material 
(Type III.) more sharply curved than when composed of a light 
material (Type I.)? 

1 See Dr. Barnard's interesting and important observations, published in No. 308 of 
the " Astronomical Journal." 



COMETS AND METEORS. 223 

4. If a comet were a compact sphere 80 miles in diameter, the 
average density of which equalled that of the earth, its mass would 
be what fractional part of the earth's mass, the earth's diameter be- 
ing called 8,000 miles? 

5. A circular shield in front of the earth, to protect it in case of 
collision with a comet, would have an area of about 50,000000 
square miles if it were 8,000 miles in diameter. Verify this statement. 
Suppose that 1,000,000000 masses of the size of a man's fist uni- 
formly distributed throughout the comet pelted the shield during 
such a collision. Is it likely that a particular house located on the 
front of the shield would be struck? 

6. If the nucleus of a comet spurts out a jet toward the sun, 
does that action tend to drive the nucleus away from the sun ? 

7. If a meteorite overtake the earth, is it less liable to be shattered 
in the air than if it meet the earth ? 

8. What evidence is there that a meteorite, before it strikes the 
atmosphere, is a cold body? 

9. On some clear moonless night observe a meteor, estimating its 
time of flight, and the direction in which it moved. Note its color 
and the length of its train (in degrees), remembering that the dis- 
tance between " The Pointers " in the Great Dipper is five degrees. 

10. Do meteors gradually increase the size of the earth? 

n. Do those meteors which meet the earth, and thus resist its 
motion, tend to lengthen or shorten the year? (§ 302.) 

12. The sun must be struck by meteors, as well as the earth. 
Do meteors tend to increase the attraction between the sun and 
the earth? 

13. If the attraction between the sun and the earth be thus in- 
creased, does the increase operate to shorten the year? 

14. Would a decrease of the distance between the sun and the 
earth operate to shorten the year? 

15. Judging by the effect of meteors on the temperature of the 
earth, do you think that the sun's heat can be accounted for bv 
their impact? 



224 



DESCRIPTIVE ASTRONOMY. 




Fig. 169. — Stars Visible to the Naked Eye. 



THE FIXED STARS. 225 



CHAPTER XII. 

THE FIXED STARS. 

" What involution ! what extent ! what swarms 
Of worlds, that laugh at earth ! immensely great ! 
Immensely distant from each other's spheres ! " 

Young. 

336. Number Visible. — The stars visible to the naked eye are by no 
means countless. Over 2,000 of them can be seen at one time, under 
favorable circumstances. Were it possible to see the entire celestial 
sphere as well as one sees the portion of it near the zenith, more than 
6,000 stars could be enumerated. There are multitudes of stars just 
below the limit of naked-eye vision which a spy-glass brings out 
without difficulty (Fig. 169). It will show twenty stars for every one 
seen without its aid. The forty-inch Yerkes telescope (Fig. 170) of 
the University of Chicago may be capable of revealing 1,000 times 
as many stars as a lady's opera-glass. As faint stars are much more 
numerous than bright ones, only a few hundred stars can be seen 
without a telescope when the full moon dominates the heavens. 

337. Scintillation. — Comparison of stars near the zenith with 
those at a greater distance from it shows that stars twinkle the more, 
the nearer they are to the horizon. Since the light coming from 
those near the horizon passes through a greater thickness of air than 
that from those at higher altitudes, it is more violently disturbed. 
The strata of air through which it passes have many degrees of 
density, so that the light is refracted hither and thither on its way to 
the eye. The image of a star in a telescope is made to boil or 
dance. The irregular refraction of the light also causes a phenome- 
non described in works on physics as " interference." In conse- 
quence of this there are continual changes of color ; the flashes of 
many colors which emanate from Sirius, the brightest of the fixed 
stars, when it is near the horizon in the early evening in midwinter, 
are very beautiful. An electric arc light scintillates when seen 
at a distance of several blocks. Scintillation is generally most 
marked on windy or frosty nights. When the stars twinkle vio- 

*5 



226 



DESCRIPTIVE ASTRONOMY. 



lently, telescopic observations are usually of little value. A little 
haze uniformly suffused through the atmosphere reduces scintillation 




Fig. 170. — The Yerkes Telescope at the World's Fair, 1893. 

to a minimum : on a slightly hazy night the stellar images seen in a 
telescope are in general small and neat, and bear magnifying well. 



THE FIXED STARS. 



227 



The planets, having much broader disks than the fixed stars, 
twinkle very little ; when one point of their disks is temporarily dark- 
ened by interference, another may become brighter, so that the total 
quantity of light from all points of the disk remains about the same. 




Fig. 171. — A Portion of the Milky Way 



338. The Milky Way. — The Milky Way, or Galaxy, is the broad 
bright stream of stars which encircles the heavens, and exhibits a 



2 28 DESCRIPTIVE ASTRONOMY. 

fine contrast with the blackness of the sky at times when the moon 
is not visible. The Galaxy is not of uniform brightness ; in places 
there are striking dark spots, many of which look like vast abysses : 
one in the constellation of the Centaur is called the Coal Sack : in 
Cygnus there is another starless space, smaller than the Coal Sack. 
The portion of the Milky Way seen south of the zenith in middle 
north latitudes in midsummer is divided into two streams, lying side 
by side. The sheen of the Galaxy is due to the fact that it is com- 
posed of millions of closely packed faint stars. In the brighter por- 
tions of it, hundreds of stars can be seen in the field of view of an 
opera-glass. Photographs taken with large lenses and long expos- 
ures show that there is a marvellous complexity of structure ; there 
are sprays of stars, and vast cloud-like appearances, 1 which are 
crossed by dark lanes and bestrewn with dark spots. 

339. Tree-like Structures. — Many solar prominences have tree-like 
forms : one is astonished to find such forms in the Milky Way, but 

photography gives indubitable evidence of their exist- 
ence. Some of these are dark, and others are bright. 
Fig. 172 represents a dark plant-like structure near 
Alpha Cygni, which appears on a photograph taken 
by Dr. Wolf 2 with an exposure time of eleven hours. 
It has been conjectured that these forms are due to 

^jjgP* 5 "^ colossal uprushes into a resisting medium. But the 
1 dimensions of these mysterious objects are so enor- 

jf^r- mous that this explanation seems inadequate. The 

■*£=----** = matter is still further complicated by the fact that 

Fig. 172. — Plant- m any f the dark structures are bordered by lines of 
like Structure. 

stars. 

340. The Constellations. — In the early ages men grouped the 
brighter stars fantastically, and gave names to the groups. The 
Latin forms of these names are now employed. Some of the most 
commonly known constellations visible in the United States are 
Cassiopeia (the Lady in the Chair), Cygnus (the Swan), Leo (the 
Lion), Lyra (the Lyre), Orion, and Ursa Major (the Great Bear). 
Most of the constellations are named after mythological personages, 
or after animals. Ptolemy, who died A. D. 170, revised the scheme 

1 Dr. Barnard first photographed these. 

2 Dr. Max Wolf, of Heidelberg, one of the foremost of celestial photographers. 




THE FIXED STARS. 229 

of constellations known to the ancients and transmitted to us forty- 
eight of them. More modern astronomers have added a large num- 
ber of other constellations to fill in the spaces not covered by the old 
ones. Nineteen of these are now generally recognized. 

341. Names of Individual Stars. — Many of the brighter stars have 
received proper names, drawn from the Latin, Greek, and Arabic 
languages. Such are Sirius, Polaris, Rigel, Aldebaran, and Vega. 
Practical astronomers use only a few of them. Such names as 
Skat, Rotanev, Muphrid, and Zavijava are sinking into deserved 
oblivion. 

Naked-eye stars are most commonly designated by letters or 
numbers prefixed to the genitive case of the Latin name of a con- 
stellation. The general plan is to call the brightest star in a given 
constellation by the Greek letter Alpha, the next brightest being 
Beta, and so on through the alphabet. Thus Alpha Lyrae is the 
brightest star in Lyra. After the Greek alphabet (§ 405) is ex- 
hausted, the Roman is used. Thus we have such names as Delta 
Herculis and/ Herculis. The system of Greek and Roman letters 
does not follow the order of brightness accurately. There is a sys- 
tem of numbering which is independent of the other two, the stars 
of a constellation being numbered according to the order in which 
they cross the meridian. For instance 1 Orionis crosses before 
2 Orionis. A star may have both a letter and a number, the former 
being preferred: Eta Aurigae is the same star as 10 Aurigse. 

For telescopic stars the names are taken from catalogues. 
Lalande 19,486 is the 19,486th star in Lalande's catalogue of 
stars (§ 344). 

342. Orders of Brightness. — Stars visible without telescopic aid are 
divided into six orders of brightness, called magnitudes (§ 1). Stars 
of the sixth magnitude are just perceptible by an ordinary eye : two 
thousand of them are visible in the United States. A few of them 
may be seen within the bowl of the Great Dipper. Those of the 
fifth magnitude are plain to persons who are not short-sighted. The 
uppermost of the three distinct stars in the sword-handle of Orion 
is of the fifth magnitude. There are twenty stars of the first magni- 
tude, fourteen of which lie between the north pole and 35 of south 
declination (§ 122), and can be seen from any point in the United 
States. Polaris is of the second magnitude. An average star of 



23O DESCRIPTIVE ASTRONOMY. 

the first magnitude is one hundred times as bright as one of the 
sixth magnitude. 

343. Magnitudes of Telescopic Stars: Ratio of Magnitude. — The sys- 
tem of magnitudes outlined in the preceding section is extended to 
telescopic stars. Of late years a uniform ratio of brightness between 
stars of successive magnitudes has been adopted by astronomers ; it 
is called the "light ratio." A star of a given magnitude is vioo 
times as bright as a star of the next lower magnitude: V 100 = 2.5 
nearly. A star of the third magnitude is 2.5 times as bright as one 
of the fourth. The forty-inch telescope mentioned in § 336 should 
reveal stars of the seventeenth magnitude. Stars even fainter than 
this can be photographed. The brightness of a star the magnitude 
of which is between two integral magnitudes is expressed by the aid 
of decimals. Thus a star of the 6.4 magnitude is fainter than one of 
the sixth, and brighter than one of the seventh magnitude. 

344. Star Catalogues. — An observer with a meridian circle and a 
clock can determine the right ascensions and declinations of a large 
number of stars, as explained in Chapter VI. These, when arranged 
in the order of their right ascensions, constitute a star catalogue. 
The names of the stars and their magnitudes are also given. A 
small catalogue is given each year in the Nautical Almanac. Each 
of the star places given in this catalogue depends upon hundreds 
of observations. 

345. Photographic Star Charts. — Stars whose right ascension and 
declination have been found can be charted, but the work is very 
laborious. Charts are made much more expeditiously by the use 
of photographic plates. A number of observatories, scattered 
throughout the world, are now (1896) engaged in securing photo- 
graphs of the entire sky ; the plates will exhibit millions of stars 
which have never been catalogued. 

Prof. E. C. Pickering has planned a similar campaign with the 
Bruce photographic telescope. This instrument should do such 
work with much greater rapidity than others hitherto constructed. 
Its objective is a quadruple lens, two feet in aperture. 

346. Distribution. — The stars visible to the naked eye are distrib- 
uted over the face of the sky with tolerable uniformity. This is 
shown in Fig. 169. When telescopic stars are taken into consider- 
ation the case is very different. These are massed in great profu- 



THE FIXED STARS. 23 I 

sion in and near the Milky Way. The farther one goes from the 
Galaxy, the fewer the stars become. This fact was established by 
the observations of the Herschels, father and son, who pointed a 
large telescope equipped with a certain magnifying power to a few 
thousand different places in the sky, and counted the number of 
stars visible in the field of view each time. In the Galaxy, 122 stars, 
on the average, were visible at one time in the field of view; 15° 
from the Galaxy, only thirty were similarly brought into view; at 
a distance of 30 , only eighteen were seen; at 45 , ten were 
counted ; 90° away, the average number sank to four. There is here 
a resemblance to the distribution of vegetation on the earth : it is 
most luxuriant at the equator, and diminishes as one goes toward 
the poles. 

347. Clusters. — The unassisted eye reveals several coarse clusters, 
of which the best known is the Pleiades, in the constellation Taurus. 
The Hyades in the same constellation, Prsesepe in Cancer, and the 
cluster in the sword-handle of Perseus are plain to the naked eye, 
and will well reward the trouble of looking at them with an opera- 
glass. In some telescopic clusters, the stars crowd upon one 
another so closely that the telescope cannot separate them ; near 
the centre of the well known cluster in Hercules star crowds upon 
star in a blaze of glory (Fig. 173). When looking at one of the 
closely packed clusters with a large telescope, one is apt to get the 
impression that he is gazing across measureless vistas of space at a 
remote system of stars, which appear faint and crowded together 
on account of their stupendous distance. But such is not the case. 
The best evidence at command renders it practically certain that 
their distances from us are no greater than those of more scattered 
stars. Though the stars in a given cluster are supposed to be near 
enough to one another to be subject to considerable mutual attrac- 
tion, no motion due to such a force has yet been detected. Their 
motions will probably be recorded on the photographic plates in 
due time. 

Ranyard x held that there is evidence that collisions are taking 
place between the stars, as a result of their mutual attraction. 
If two such bodies collided, the rapid evolution of heat at their 
point of contact would expand the contiguous gases so violently 

1 The late Mr. A. C. Ranyard, who was editor of " Knowledge," London. Lug. 



232 



DESCRIPTIVE ASTRONOMY. 



that the effect of an explosion would be produced, and the stars 
might rebound like caroming billiard balls, while the gases heated 
at the point of impact would diffuse themselves in the surrounding 
space. To this he attributed the radiated appearance of the outlying 
stars, which are frequently arranged in streams, as if ejected from 
the central mass. The stars in a stream are often connected by a 
band of nebulous matter. Photography has revealed the fact that 
nebulosity is associated with very many star clusters. 




Fig. 173. — The Great Cluster in Hercules. 



348. Dimensions and Nature of the Stars. — The larger the magnify- 
ing power employed, the larger does a near object, like the moon or 
a major planet, appear. The diameters of these bodies have been 
measured. But with the fixed stars the case is entirely different. 
The larger and more perfect the telescope, the smaller the disk of a 
star, under the best atmospheric conditions. The visible disk is a 
spurious one, the cause of which is explained in works on optics. 



THE FIXED STARS. 



233 



The diameter of the sun, as seen from the nearest fixed star, is 
equivalent to that of a small marble a thousand miles from the ob- 
server. Though the diameters of stars subtend so small a visual 
angle as to defy direct measurement, yet the spectroscope has given 
us a little knowledge concerning them. The star Algol (§ 379) 
has been shown to be over a million miles in diameter: it has an 




Fig. 174. 



The Cluster Omega Centauri : Photographed by Dr. Gill, 
at the Cape of Good Hope. 



invisible companion of nearly the same size as the sun. Arcturus, 
Capella, and Vega are believed to be much larger than our sun : 
the diameter of Arcturus may be a hundred times that of the sun. 
The second magnitude star in the crook of the handle of the Great 
Dipper (Zeta Ursae Majoris) is forty times as massive as the sun. 
The spectroscope has also shown that the stars are self-luminous 
bodies, similar to the sun. 



2 34 



DESCRIPTIVE ASTRONOMY. 



349. Distances. — The distances of the stars are inconceivably 
great, though easily expressed in figures. Alpha Centauri is nearer 
than any other star the distance of which has yet been measured. 
Its distance from us is 275,000 times the distance of the earth from 
the sun ; light consumes over four years in traversing this distance. 
Sirius, the Dog Star, is twice as far away. Most of the stars are at 
distances so stupendous as to defy measurement. It is considered 
probable that the vast majority of the stars are so far away that their 
light occupies over a century in coming to us. The light which 
reaches our eyes from many a one of them, may have been on the 
journey for thousands of years. Less than one hundred stars have 
been found to lie within measurable distances. 

In estimating such stupendous distances it is convenient to em- 
ploy as a unit the " light-year," that is, the distance over which light 
would travel in a year. A locomotive with driving-wheels 60 miles 
in diameter, which were revolving 1,000 times a second, would trav- 
erse the distance in a few days less than a year. 



EARTH 




STAR 



Stellar Parallax. 



350. Stellar Parallax. — The annual parallax of a star is the 
apparent semidiameter of the earth's orbit as seen from the star. 
This angle, formed at the star by the two lines shown in Fig. 175, is 
very small. In the case of Alpha Centauri, our nearest neighbor, 
the parallax is only as great as the apparent diameter of a sphere 
one foot through, located fifty miles from the observer. 




FAINT . . 
5TAR5 v 



Fig. 1; 



Method of observing Stellar Parallax. 



The method most employed for determining parallax is illustrated 
in Fig. 176. Usually some stars much fainter than the star the 



THE FIXED STARS. 



235 



parallax of which is sought can be seen in the same telescopic field 
of view with it. It is reasonable to suppose that these are much 
further away. When the earth is at E the star A will appear to be 
situated at the point X among the fainter stars. Six months later, 
when the earth is at E', 186,000000 miles from its former position, 
the star A will appear to be located at Y. This apparent shift of 
position, when accurately measured, gives an astronomer the means 
of computing the star's parallax. The explanation of the methods 
of observation and calculation by which the parallax is found lies 
beyond the scope of this book. The parallax being known, it is a 
simple matter to find out how far away the star is. For the semi- 
diameter of the earth's orbit is 93,000000 miles, and the parallax is 
the angle at the star in Fig. 175. The problem then is to find 
how far from the star a line 93,000000 miles long must be, in order 
to subtend a visual angle equal to the star's parallax. Its solution is 
given in the next section. 




Fig. 177. — Relation of Parallax to Distance. 



351. Solution of the Problem. — In Fig. 177 let the star be at A, 
the sun at S, and the earth at E. Then the angle SAE is the 
star's parallax ; this angle is so minute that the arc SE is practically 
of the same length as its chord, and we may use d to designate 
either of them. Represent the distance AS by R, and let p equal 
the number of seconds in SAE. The circumference of the circle of 
which SE is an arc is 2 it R. There are 360 , or 1,296000", in the 
circumference; hence the length of an arc of 1" is 1 2 2 9 g^ 00 > which 
equals 2 Q ^ 2 . The length of an arc of p seconds equals p times 
this expression. 

Hence d=^^, whence R = ^^J. 



236 DESCRIPTIVE ASTRONOMY. 

According to this formula, if any star had a parallax as large as 
five seconds, its distance from the sun would be 2 ° 6 . 2 6 5 , or 41,253 
times the earth's distance from the sun. 

352. Colors. — Most of the stars are white : there are many of a 
yellowish or reddish tinge. A few are of very pronounced colors : 
Sirius is white ; Vega has a bluish tinge ; Arcturus is reddish. A 
few faint stars are deep red. Those which are close to brighter ones 
are usually bluish or greenish. 

It was once thought that the color gave a clue to the temperature 
of a star, the white stars being much hotter than the red ones, just 
as white-hot iron is at a higher temperature than red-hot. But this 
theory is now abandoned : the colors are probably dependent on 
the materials which enter into the composition of the star, as well 
as its temperature. The references to the color of Sirius by ancient 
writers render it highly probable that it was red at the beginning of 
the Christian era. 

353. Spectra. — Secchi, an Italian astronomer, divided stellar 
spectra into four arbitrary classes, or types. 

Type I. The dark lines due to hydrogen are very pronounced ; 
other lines are few and inconspicuous. This type embraces the 
majority of the stars ; their colors are white or bluish. They are 
called Sirian stars, as Sirius belongs to the group. 

Type II. The spectrum resembles that of the sun, being crossed 
by numerous dark lines, indicating the presence of various metals. 
These are called solar stars, and their colors are mostly yellow. 
Our nearest neighbors among the stars have recently been shown to 
belong to this type. 

Type III. In spectra of this type, shaded bands are seen, each 
of which is darkest at the edge nearest the violet end of the 
spectrum, and shades off toward the red end. The color of these 
stars is orange or red. 

Type IV. As in Type III. we have here a banded spectrum, but 
the bands are darkest at the edge nearest the red end of the spec- 
trum, and shade off toward the violet end. These stars are faint, red, 
and few in number. 

To these a fifth class is now added, embracing the so called 
" Wolf-Rayet " stars, which have bright line spectra. More than 
fifty of these are known. 



THE FIXED STARS. 237 

354. Discussion of Stars of Different Spectral Types. — Two thirds 
of the Sirian stars are situated in the Milky Way, while the solar 
stars are about evenly divided between galactic and non-galactic 
regions. Each square mile of the surface of a Sirian star is 
brighter than an equal area of a solar star, but solar stars are on 
the average much more massive than Sirians, and give a greater 
quantity of light. 

The Wolf-Rayet or " bright-line " stars lie in or near the Milky 
Way : these stars are of special interest, because they apparently 
form a connecting link between the nebulae (§ 388) and other stars. 
Bright lines are not uncommonly found in the spectrum of the sun 
itself, and are thought to be due to masses of vapor hotter than the 
underlying photosphere. 

The stars in Orion (with the notable exception of Betelgeuse) 
have a special variety of spectrum, scarcely found outside of that 
constellation. This indicates that these stars have a similar structure ; 
probably they are " chips off the same block." 

" In general, it may be stated that, with a few exceptions, all the 
stars may be arranged in a sequence, beginning with the planetary 
nebulae (§ 385), passing through the bright-line stars to the Orion 
stars, thence to the first type stars, and by insensible changes to 
the second and third type stars. The evidence that the same plan 
governs the constitution of all parts of the visible universe is thus 
conclusive." 1 

Different spectra doubtless indicate, in many cases, different stages 
of evolution, but many more observations must be made before any 
far reaching theory can be suitably fortified. 

355. Light and Heat. — The amount of light received from some of 
the stars has been compared with that given us by the sun. Though 
Sirius far outshines any other fixed star, being nearly six times as 
bright as Vega, 7,000,000000 stars like it would be required to fur- 
nish daylight; 9,000000,000000 stars of the sixth magnitude would 
be necessary for the same purpose. 

Professor Young estimates that the full moon gives sixty times as 
much light as the entire starry sphere ; and that ninety-five per cent 
of the latter comes from stars invisible to the naked eye. 

1 Prof. E. C. Pickering, in " Astronomy and Astrophysics" for October. i$9j. 



238 DESCRIPTIVE ASTRONOMY. 

No trustworthy measures of the heat reaching the earth from any 
particular star have been made. It is too small to affect the most 
delicate thermometric appliances. 

356. Bird's-eye View of the Stellar System. — The following conclu- 
sions have been reached by a study of the star gauges made by the 
Herschels, assuming that the faint stars are, as a class, more distant 
than the bright ones. Though subject to considerable uncertainty, 
they are generally given in works on descriptive astronomy. 

(a) Most of the stars are not arranged in the form of a sphere, 
but in that of a thin disk ; the shape of the disk is about that of a 
silver dollar. 

(b) Only a small proportion of the stars lie on one side or the 
other of the disk, but the majority of the nebulae find their homes 
there; i. e. outside of the disk. 

(c) Within the disk are the stars embraced in the Milky Way, 
which contains most of the Sirian stars. 

(d) The stars are not evenly distributed throughout the disk. 
The fainter ones are grouped in clusters and streams, as a nation is 
divided into families. Many of the brighter stars are thus grouped, 
but each " family " consists of fewer individuals than in the case of 
the faint ones. 

(e) The sun lies near the centre of the disk. 

357. Kapteyn's Investigations. — Prof. J. C. Kapteyn, a Dutch as- 
tronomer, has made the most exhaustive discussion of the form of 
the sidereal universe. The most interesting of his conclusions may 
be summed up under three heads. 

(a) The nearer stars are chiefly of the solar spectral class, and are 
scattered about the sun on all sides, independently of the position of 
the Milky Way. They form with the sun a scattered cluster. 

(#) Those stars the distances of which from us are immeasurably 
great, whether Sirian or solar, are more numerous the nearer they 
lie to the plane of the Milky Way. 

(c) Of the stars of any given brightness (say sixth magnitude), 
those which lie in or near the Milky Way are, on the whole, more 
remote from us than those which lie in other parts of the heavens. 

The stellar universe thus bears a rude resemblance to the planet 
Saturn, consisting of a central ball of stars, surrounded at a great 
distance by an apparently ring-shaped collection of stars. Professor 



THE FIXED STARS. 239 

Kapteyn likens it to the nebula in Andromeda (§ 389 , the cen- 
tral nucleus of which corresponds to the solar cluster, while the out- 
lying whorls are miniatures of the Galaxy. 

358. Proper Motions. — Though the stars are called "fixed," they 
are far from being so. They are moving with various degrees of 
rapidity in all conceivable directions; but on account of their pro- 
digious distances from the earth, their positions with reference to 
one another change by minute amounts only, from year to year. 
Proper motion is this apparent shifting of a star's position on the 
celestial sphere. 

The proper motion is not the star's real motion in space. If the 
earth were at rest, and a star were coming directly toward it or going 
directly away from it, the star would appear to be fixed on the 
celestial sphere, and would have no proper motion. The largest 
proper motion yet discovered is that of the star Groombridge 1830, 
which has been graphically termed the " runaway star." In 270 
years it moves over a space equal to the apparent diameter of the 
moon. The bright stars have, on the average, larger proper motion 
than the faint ones. This is probably due to the fact that their 
average distance from us is less than that of the faint stars. Arc- 
turus has apparently moved over a space equal to one fifth of the 
distance between the Pointers in the Great Bear, during the Chris- 
tian era. 

It has been shown, by combining a mass of observations on numer- 
ous stars, that the average proper motion of a first magnitude star is 
six times that of one of the sixth magnitude. Stars having a large 
proper motion are at less distances from us, on the whole, than 
those of small proper motion. 

359. Proper Motion Groups. • — While proper motions have all sorts 
of directions, there are many groups of stars the components of 
which move in the same direction. The stars in the Great Dipper, 
with the exception of the one at each end of the figure, belong to 
such a group. By spectroscopic observations (§ 360) it has been 
found that these five stars are all retreating from us to greater depths 
of interstellar space. They are separated from one another by 
distances inconceivably vast. Yet there seems to be some common 
bond, and the laws of motion of this stupendous system ma}' for- 
ever elude the keenest search of man. 



240 



DESCRIPTIVE ASTRONOMY. 




Fig. 178. — Proper Motions of the 
Pleiades. 



The Pleiades constitute a similar system. Of the four hundred 
stars catalogued in this cluster, only a few refuse to conform to a 

common proper motion pos- 
sessed by the others. These 
outre stars are probably be- 
tween us and the cluster 
proper, or beyond it. The 
other stars all agree in hav- 
ing the same spectra. 

360. Motions in the Line of 
Sight. — Every star is con- 
stantly sending forth lumi- 
nous vibrations of various 
wave lengths. If the star be 
approaching, the number of 
waves which reach us in any 
given time is increased, and 
their wave length is shortened. Hence, when a star is coming to- 
ward us, its light is rendered more refrangible, and all of the lines 
in its spectrum are shifted toward the violet end of the spectrum. 
By comparing the position of the hydrogen lines, for example, in 
the spectrum of the star, with the spectrum of hydrogen obtained 
in the laboratory (§ 73), the shifting of the lines can be measured. 
The velocity of approach or recession of the star can then be com- 
puted, due allowance being made for the earth's motion. 

The majority of the stars yet observed in this way exhibit velocities 
of less than thirty miles a second. The best modern results in this 
line of work are not subject to errors exceeding a mile a second. 

361. The Sun's Path. — A man is in a boat on a small lake sur- 
rounded by a forest. The boat is drifting, he knows not whither. 
He watches the trees carefully, and finally perceives that the trees 
at his right appear to be spreading apart and growing taller. Those 
on the left seem to be crowding more closelv top-ether. He can de- 
tect no change in the relative situations of the trees ahead of him, or 
those behind him. He at once decides that his boat is drifting 
toward the right. 

In this manner astronomers have discovered the direction in 
which the sun with its attendant planets is drifting. While the 



THE FIXED STARS. 24 I 

proper motions of the stars are in all directions, when we combine 
large numbers of them in a single discussion, a prevailing common 
drift comes out clearly. The stars in the constellations Lyra and 
Hercules are slowly separating from one another. Those in the 
opposite part of the sky are crowding together. The proper mo- 
tions are so small that it is not possible to fix the point toward 
which we are moving with much precision. Spectroscopic observa- 
tions of the velocities of stars make the sun's velocity only from 8 to 
12 miles a second. 

362. The Central Sun. — There is a persistent idea that there is a 
central sun. One theory, which has obtained a wide currency, is 
that Alcyone, the brightest of the Pleiades, is the central sun. This 
theory arose fifty years ago from a study of the proper motions of 
the Pleiades. In the light of our present knowledge concerning 
proper motions, the theory is considered untenable. 

The hypothesis that the sun is sweeping around a gigantic curve 
is a reasonable one ; but no deviation from a straight line has yet 
been detected in its motion. Even if the centre of its motion be 
found, it by no means follows that all the other bodies in the uni- 
verse move about that centre. 

363. The System of the Stars. — Evidences of order and obedience 
to law are so numerous in the entire domain of physical science, 
that the human mind instinctively seeks for some law or set of laws, 
in accordance with which all the stars pursue their journeyings. 

The only law now known, which the motions of the heavenly 
bodies follow, is that of gravitation. But while there are many 
systems more or less similar to the solar system among the stars, 
each is so far from its neighbors that it experiences very little attrac- 
tion from them. They exist in great variety, from the largest and 
most complicated clusters, down to simple double stars ; their con- 
nection with one another is only a matter of conjecture. It seems 
very probable that there is no central sun, or even central point, 
about which the universe moves in orderly fashion. 

The solar system is a fairly well regulated family. The stellar 
system seems to be made up of families and tribes which are largely 
independent; while each family or tribe exercises some influence 
upon the neighboring ones, it apparently attends pretty strictly to its 
own affairs. 

16 



242 



DESCRIPTIVE ASTRONOMY. 



DOUBLE AND MULTIPLE STARS. 

364. Appearance to the Naked Eye. — By surveying the heavens 
for a few minutes one may find several places where two stars lie in 
close proximity to each other. Theta Tauri, in the head of the Bull, 
and Alpha Capricorni, are good examples. But neither of these is 
ordinarily classed as a double star, for the components of each pair 
are too far apart. The stars which make up a real " double " are so 
close together that a telescope is required to separate them. 

385. Appearance through a Telescope. — Fig. 179 exhibits some of 
the double stars, when seen under a high magnifying power. When 



•• 

GAMMA LEONIS 


• 

• 

BETA CYGNI 


• • 

ALPHA HERCULIS 


ANTARES 



Fig. 179. — Double Stars. 

the two stars are of the same brightness they are also of the same 
color. When they differ considerably in brightness the smaller star 
is apt to have a bluish cast. Beta Cygni, in the foot of the Cross, is 
one of the finest of those colored doubles which can be seen with a 
small telescope. The large star is reddish yellow, the small one 
greenish blue. The colors may be seen beautifully, by putting the 
stars out of focus. The two stars are often so close to each other 
that even a very powerful telescope shows them bunched together 



THE FIXED STARS. 243 

in an oblong mass of light. At times the blaze of a bright star quite 
overpowers the feeble light of its faint companion. 

366. Numbers and Nomenclature. — The number of doubles thus 
far catalogued is over 10,000. New ones are being discovered con- 
tinually, but not at a rapid rate, since there are few stars above 
the eighth magnitude which have not been scrutinized carefully with 
large instruments. Doubles, the principal stars of which are no 
brighter than the ninth magnitude, are rarely catalogued. 

Each double retains its ordinary name, such as Sirius, Gamma 
Virginis, 61 Cygni, etc., and acquires an additional one taken from 
the name of the discoverer. Thus h 1064 is one of Sir John 
Herschel's discoveries : ft 462 was found by Prof. S. W. Burnham 
of Chicago, the greatest living double-star astronomer. 

367. Optical Double Stars. — An optical double star is one the 
components of which seem to be near each other, but are not; one 
of the stars lies far beyond the other. Optical pairs form but a very 
small percentage of doubles. They are detected by the absence of 
such motion as would ensue were the stars so near together that 
their mutual attraction caused relative motion. 

368. Physical Double Stars. — The stars forming a physical double 
are subject to the sway of their own mutual attraction. Observa- 
tions of them reveal the fact that they move in elliptical orbits. 
This leads to the belief that gravity is the force which controls their 
motions. A force acting according to some other law might pro- 
duce elliptical motion, as is proved in works on mechanics. But 
since the spectroscope shows that the stars are composed of much 
the same materials as the sun, it is reasonable to suppose that their 
attractions for one another follow the same law which holds good 
in the solar system. Gravitation may therefore be considered as 
universal. 

Physical double stars are usually termed binaries. Many of the 
periods of revolution are hundreds of years in length ; a few are less 
than a year. Some of the orbits are several times as large as that of 
Neptune. Others are smaller than that of Mercury. 

369. Spectroscopic Binaries. — When the components of a binary 
are so close together that the most powerful telescopes fail to 
separate them, or to give any indication that the star is double, the 
spectroscope in a few instances has revealed the duplicity. The star 



244 DESCRIPTIVE ASTRONOMY. 

Mizar at the crook of the handle of the Great Dipper is a case in 
point. A small telescope easily resolves this into two stars. Prof. 
E. C. Pickering, in 1889, found by his photographs of this double, 
that the spectrum of the brighter component exhibited strange 
anomalies. At regular intervals of a few weeks the dark lines in the 
spectrum were doubled. The explanation of this depends upon 
the principle (§ 360) that when a star is approaching us the lines of 
its spectrum are shifted toward the violet, and when it is receding 
the lines are shifted toward the red. When two bright stars, close 
together and composed of the same substances so that they give 
the same spectra, are revolving about their common centre of gravity 
in an orbit the plane of which is nearly edgewise to us, one star will 
be approaching when the other is receding. Were the stars at rest, 
their spectra would coincide in position. But when, on account of 
their motion, the lines in one spectrum are shifted in one direction, 
and those in the other in the opposite direction, the lines which 
formerly coincided will appear side by side. 



EARTH 



Fig. 180. — A Spectroscopic Binary. 

W T hen the stars are at A and B in Fig. 180, they are neither 
approaching the earth nor receding from it so far as their orbital 
motion is concerned. 

The two close stars in Mizar complete a revolution in one hun- 
dred and four days, 1 in an orbit of the same size as that of Mars. 

Spica, in Virgo, is a yet more wonderful double. The compo- 
nents are only about 6,000000 miles apart, and complete a revolution 
in four days. 

370. Sirius. — Certain minute movements of Sirius on the face of 
the sky, hither and thither, were for a long time a source of perplex- 
ity to astronomers. Fifty years ago the illustrious German astron- 
omer, Bessel, announced that the observations of Sirius indicated 
that it was describing a tiny ellipse. He also advanced the theory 
that the motion was caused by the proximity of a companion. Less 

1 Possibly in just half that time. 




THE FIXED STARS. 



245 



than ten years thereafter, two other German astronomers declared, 
as the result of an elaborate investigation, that the period of orbital 
revolution of Sirius and his unseen satellite was fifty years ; they also 
pointed out the direction in which the companion lay from the larger 
star, and the direction of its motion. Eight years later, when the 
Clarks were testing an 18 J- inch object-glass, they turned it upon 
Sirius. The keen eye of Alvan Clark, Jr. quickly detected a faint 
star in the blaze of light surrounding the large star. It was soon 
found to be moving in the way predicted. The mass of the system 
is six times that of the sun. The faint star, which gives less than 
* as much light as the main star, may contain one third of the 
mass of the system. 

371. Planetary Systems. — As the sun is the ruler of a planetary 
system, many of the stars may be centres about which troops of 
planets roll and shine. Such planets, in order to be discovered by 
us, must be much more brilliant in comparison with their suns than 
are those of the solar system. Jupiter himself, if searched for from 
Alpha Centauri with the most powerful telescope ever constructed 
by man, would elude the most searching scrutiny. Professor Young 
has computed that a refracting telescope ten feet in aperture would 
be needed. Such companions, if not too near their primaries, may 
in the future impress themselves on photographic plates of great 
sensitiveness. 

372. Multiple Stars. — Epsilon Lyrae is a fine specimen of a 
multiple star. It is one of the two fourth magnitude stars near Vega 






EPSILON LYRAE THETAORIONIS ZETA CANCRI 
Fig. 181. — Multiple Stars. 

which form with it an equilateral triangle. To a good eye the star 
appears oblong; a keen eye separates it into two. An opera-glass 
shows them finely. A telescope three inches or more in aperture 



246 DESCRIPTIVE ASTRONOMY. 

reveals each star as a double. We have, therefore, a quadruple 
star. Each pair is a binary : it is probable that the two pairs revolve 
about their common centre of gravity, completing a single revolution 
in many thousands of years. 

Theta Ononis is composed of six stars. It is located in the 
sword-handle of Orion, and is involved in the great nebula of Orion 
(§ 39°) • There is good evidence that these stars have been formed 
by the condensation of a portion of the nebula. 

Zeta Cancri is visually a triple star, two being close together, the 
other farther away. The close pair is a binary, and the third star 
apparently revolves about the binary, but with singular irregularities 
of motion. The irregularities are thought by some astronomers to 
be due to a fourth star near by, but invisible. The system is in 
that case composed of two binaries, which revolve about their com- 
mon centre of gravity. 

VARIABLE STARS. 

373. Definition : Number : Names. — Variable stars are those the 
brightness of which has been observed to change. Those that repeat 
the same series of changes over and over are known z& periodic, the 
period being the time required for the star to pass through one com- 
plete cycle of change. Some naked eye stars become too faint to 
be seen with a telescope four inches in aperture. New variables 
are discovered from time to time, and the number now (1896) 
well authenticated is 400. This number does not include those 
variables which were discovered in 1895, x in certain globular star 
clusters; nearly 1 00 variables were noted in a single cluster. When 
such stars already have names (§ 341) other than mere numbers in 

,some star catalogue, no new name is added to denote variability. 
But when the stars are faint, so that they have not received such 
names, the first such variable discovered in the constellation An- 
dromeda, for instance, is named R Andromedae. The second would 
be S, and so on through the alphabet. After the letter Z has been 
reached, further discoveries receive the designations RR, RS, etc. 

374. Classes. — These are classified in five groups. 

Class I. embraces temporary stars, which suddenly experience an 
enormous increase in brightness, and then fade away gradually. 

1 By Prof. S. I. Bailey, at Arequipa, Peru. 



THE FIXED STARS. 



247 



Class II. includes periodic stars which suffer great variations of 
light in not less than several months. 

In Class III. are found stars which exhibit slight irregular fluctua- 
tions of brightness. 

For Class IV. are taken those stars of short periods, the light of 
which varies smoothly and regularly. 

Class V. is devoted to those periodic stars which suffer a remark- 
able diminution of light for a few hours, every few days. They be- 
have as if temporarily partially eclipsed. 




Fig. 1* 



Tycho Brahe. 



375. Temporary Stars. — One evening, in November, 1572, when 
Tycho Brahe was taking his usual walk, he perceived in the con- 
stellation of Cassiopeia a new star, brighter than Sinus, and com- 
parable with Venus at her best. Doubting the evidence of his 
eyes, he called the attention of several others to the splendid object. 



248 DESCRIPTIVE ASTRONOMY. 

For some days the star could be discerned in full daylight, and at 

night shone through light clouds which obscured all other stars. In 

December it began to wane ; at the end of that month it had become 

fainter than Jupiter. Finally, in March, 1574, it disappeared from 

view. There were no telescopes then to watch 

it further. Its color changed from white to 

yellow and red successively, and returned to 

_ white before it faded from vision. 

STAR Tycho determined its place, but his obser- 

• • vations are so rude, from lack of telescopic 

aid. that it is impossible to tell whether the 

• star was any one of half a dozen now visible 

„. - i*. t c i n tne vicinity. 

Fig. 183. — Tycho's Star j 

in Cassiopeia. Nova Aurigae (§381) belongs to this class 

of stars. 

376. Mira. — Class II. is fitly represented by Mira, which is 
Omicron Ceti. The period of this star is eleven months. Most of 
the time it is invisible to the unassisted eye, but once during its 
period it rises to its maximum brightness, which varies from the 
second to the fifth magnitude, remains there about a week, and then 
sinks more slowly back. The rise and fall together consume about 
one hundred days. During the remainder of its period it is of about 
the ninth magnitude, and can therefore always be seen with a good 
field-glass. It is visible to the naked eye about six weeks, when 
near its maximum. 

377. Class III. — To this belong Alpha Ononis and Alpha Cassio- 
peiae. Alpha Orionis is the bright reddish star in the shoulder of 
Orion. The amount of fluctuation is small, and no period or regu- 
larity of fluctuation has been found. 

378. Beta Lyrae. — This star is a good example of Class IV. It is 
of the fourth magnitude, and varies half a magnitude on each side of 
this. The period is nearly thirteen days ; during this time the star 
first reaches a maximum of the 3.4 magnitude, then sinks to the 3.9 
magnitude, next rises again to the 3.4 magnitude, and finally sinks to 
the 4.5 magnitude. These changes are thought to be due in some 
way to the action of one or more companions, revolving about the 
main star. 

379. Algol — Beta Persei was called by the Arabians Algol, which 



THE FIXED STARS. 249 

means the Demon Star; they had therefore undoubtedly noticed 
the variation of its light. Its mean period has been very accurately 
determined, and is given by Chandler as 2 d. 20 h. 48 m. 55 sec. 
During most of the time it is of the second magnitude. Its variation 
occupies ten hours, the magnitude falling to the fourth, remaining 
there for twenty minutes, and rising again to the normal amount. 

,*, 
ALGOL CAMMA /' \ 

:•-— *-.. 

AN0ROME.DA& . s BETA 

*' "• - -r' SQUARE. OF \ 

ANDROMEDAE. •.. pEGASu5 



Fig. 184. — How to find Algol. 

The cause of this sudden diminution of light has long been sus- 
pected to be the presence of a dark star revolving about Algol and 
partially eclipsing it at each revolution. The truth of this has re- 
cently been rendered nearly certain by spectroscopic observations 
(§ 360) which show that Algol alternately approaches us and recedes, 
just as if it were one component of a binary system. The following 
approximate data concerning this binary have been derived : — 

Diameter of the principal star, 1,000000 miles. 

Diameter of the dark companion, 800000 " 

Distance between their centres, 3,000000 " 

Velocity of the companion, 55 miles per second. 

Mass of the principal star, % of the sun's mass. 

Mass of the companion, f of the sun's mass. 

There are certain small inequalities in the period of variability 
which Chandler explains by the theory that the binary already 
mentioned is involved in an orbital revolution with a heavy faint 
star, in a period of about 130 years. The size of this new orbit is 
about equal to that of the orbit of Uranus. It is possible that there- 
are other bodies in the system. Algol belongs to Class V. 

380. Y Cygni. — This star is one of the most interesting of vari- 
ables : it belongs to the Algol type. Twice in every three days it has 



25O DESCRIPTIVE ASTRONOMY. 

a minimum, at which the light is one half of the maximum amount. 
This fluctuation is explained by the supposition that the star is a 
close double, the two components being equal in size and brightness, 
and revolving in a plane which is turned edgewise to us. Twice 
in each revolution about their common centre of gravity one star 
eclipses the other : the period of revolution is thus 72 hours. If 
we consider several successive minima, calling them first, second, 
third, etc., we find that the time from the first to the third is 72 
hours, as is also the interval between the second and fourth, but the 
interval from the first to the second is not 36 hours as would be ex- 
pected. The interval between the first and second minima may be 
32 hours, for instance : then 40 hours would elapse between the 
second and third, and the interval between the third and fourth 




LINE 



OfLVLSiON EARTH. 



Fig. 185.— Y Cygni. 

would be 32 hours again. This irregularity is accounted for by 
the assumption that the stars revolve in ellipses which lie " broad- 
side " to our line of vision, as shown in Fig. 185. When the stars 
are at A and B respectively, there is an eclipse or minimum. Be- 
tween the first and second minima the stars are describing the short 
parts (ACB and BDA) of their orbits. Between the second and 
third minima the long portions, BEA and AFB, are described. 

This simple explanation does not wholly account for the observed 
irregularities in the times of the eclipses. In 1886 each period was 
36 hours, while in 1891 the successive intervals were respectively 31 



THE FIXED STARS. 25 I 

hours and 43 hours. This anomaly can be explained upon the hy- 
pothesis that the ellipses are shifting their position with reference to 
our line of sight, the disturbance being due to the attraction of some 
neighboring unseen body. 

381. Nova Aurigse. — Nova Aurigse was discovered in the latter 
part of January, 1892, by Dr. Thomas D. Anderson of Edinburgh, an 
amateur astronomer, who was in the habit of observing with a hand 
telescope magnifying only ten diameters. It was of the fifth mag- 
nitude. Soon astronomers all over the world were observing the 
spectrum and changes of brightness of this new star. The question 
at once arose whether it had previously been a telescopic star, and 
when it first displayed itself. Fortunately photographs of the re- 
gion of sky in which it lay were at hand. On Dec. 10, 1891, six 
weeks before its visual discovery, it had impressed itself on one of 
the photographic plates exposed at the Harvard College Observa- 
tory. A photograph taken in Europe on December 8 showed no 
trace of the star. The photographic evidence shows that it was 
somewhat brighter in the latter part of December, 1891, than a month 
later, when the attention of astronomers was called to it. 

The spectrum was found to be of bewildering complexity ; there 
were fine bright lines and broad dark ones ; some of the lines were 
shifted in one direction, and others in the opposite, as if there were 
two bodies moving in widely different directions. In a few weeks it 
began to decline in brightness rapidly. On April 24 it was only of 
the sixteenth magnitude, and two days later it was hardly percepti- 
ble with the Lick telescope. 

The complexity of its spectrum led to the greatest variety of 
theories as to the cause of the outburst. By some it was attributed 
to the near approach of two bodies moving with immense speed ; 
their proximity caused great mutual disturbances of a tidal nature, 
leading to the production of enormous eruptions similar to solar 
prominences, though on a vastly greater scale. 

Another hypothesis was, that some unknown heavenly body, 
speeding along its far distant path, came into collision with a cloud 
of cosmical matter, similar to the meteoric aggregations encountered 
by the earth, but much denser. Photography has revealed the pres- 
ence of such clouds (§ 338) in the Milky Way, and the Nova, like 
most temporary stars, was situated in the Galaxy. 



252 DESCRIPTIVE ASTRONOMY. 

But another strange chapter is to be added to the history of this 
remarkable object. In August it was found to have brightened up, 
having attained the tenth magnitude. It then appeared like a small 
star surrounded by a nebulous atmosphere, and its spectrum was that 
of a planetary nebula (§ 385). It still (1896) retains this appearance. 
It is not improbable that this wonderful object is at so stupendous 
a distance that all these changes occurred before the astronomers 
who have observed them were born. 

The amount of light given out, when the Nova was at its best, 
may have been many times greater than that radiated by the sun. 

382. Causes of Variability. — There have been many theories upon 
this topic. The variability of stars of the Algol type is well ex- 
plained by the hypothesis of eclipses by unseen bodies revolving 
about the variables. 

The behavior of many variables can be explained by the hypoth- 
esis that they have spots, like the sun's, though much larger, 
and that these spots have their times of maximum and minimum 
frequency, as do the solar spots. If a star had one or more large 
companions revolving about it, their attractions might cause consid- 
erable tidal disturbances, which would give rise to variability. 

The sudden appearance of temporary stars may be explained by 
terrific outbursts of heated vapors, analogous to the solar prom- 
inences. Lockyer has advanced the theory, that the variables are 
not single masses, but are rather compact swarms of meteoric 
bodies, attended by satellite swarms revolving in very eccentric or- 
bits. The satellite swarms are supposed, when nearest the main 
swarm once in every revolution, to collide with its outlying meteors, 
thus producing a temporary increase of light. 

Much research must yet be made before the complex phenomena 
exhibited by variable stars can receive any adequate explanation. 

383. How to Observe Variables. — The observations of the varia- 
tions of these stars in brightness often do not require the use of 
any telescope larger than an opera-glass. When a star is near its 
maximum or minimum, it is compared with adjoining stars of nearly 
the same brightness : it is noted as being equal in brightness to 
some particular one of its neighbors, or a trifle brighter or fainter 
than others, at a given time. The object of the observations is to 
determine the time of maximum or minimum brightness. 



THE FIXED STARS. 253 

The approximate times are given in various publications, 1 for the 
observer's guidance. Most of the observations made in this country 
on these interesting objects are by amateur astronomers. 



EXERCISES. 

384. 1. With an opera-glass or spy-glass look at some portion 
of the sky, which appears to the naked eye to be barren of stars, and 
count the number in the field of view. 

2. On a night when the moon is not shining, direct an opera-glass 
or spy-glass toward some bright spot in the Milky Way, and find 
out whether the light from that particular locality is due to a number 
of faint stars, or to a few brighter ones. 

3. On a night when the moon is not shining, find a dark spot in 
the Milky Way, and make a drawing showing its location among 
the stars. 

4. Observe five of the brightest stars visible at any given hour, 
and write down the name of each, together with its color. 

5. Estimate the color of Zeta Ursae Majoris (Mizar), and of the 
minute star (Alcor) close by it. 

6. What is the origin of the name Groombridge 1830? (§ 341.) 

7. Count the stars visible to the naked eye inside the bowl of 
the Great Dipper, when the moon is not shining, and the Dipper is 
not low down. 

8. The light of a sixth magnitude star is equivalent to the com- 
bined light of how many of the eighth magnitude? 

9. If a cluster were spherical in form, and the stars distributed 
uniformly through it, would it appear to be more condensed near 
the centre than at the edge? 

10. The intensity of light varies inversely as the square of the 
distance ; that is, if two equal lights are at distances of one mile 
and three miles from the eye, the farther one would not look one 
third as bright, but one ninth as bright. If a given star were placed 
at half its present distance from us, it would look how man}- times 
.as bright as before? 

1 The times of minima of variables of the Algol type are given in " Popular Astron- 
omy," every month. For methods of observation, see articles by Mr. J. A. Parkhurst in 
" Popular Astronomy " for December, 1893, an( ^ January, 1894. 



2 54 



DESCRIPTIVE ASTRONOMY. 



1 1. The semidiameter of the earth's orbit being 93,000000 miles, 
how far off is a star which has an annual parallax of one tenth of a 
second of arc? 

12. Show that the time required for light to come to us from a 
star having a parallax of one hundredth of a second of arc is over 
three hundred years. 

13. Do Sirian stars have atmospheres of large absorptive 
power? 

14. Do the spectra of solar stars indicate that they are probably 
more dense than Sirian stars or less dense? 

15. Do the spectra of the Wolf-Ray et stars show that they are 
surrounded by extensive atmospheres, which absorb the rays com- 
ing from within? (§ 73.) 

16. Show that the sun, if removed to the distance of Sirius, 
would appear to be less than one fortieth as bright as the latter. 

17. If the stellar system were in the form of a sphere, throughout 
which the stars were distributed uniformly, and we were at its centre, 
would the stars appear to be uniformly distributed over the face of 
the sky? 

18. Does the fact that there are many more stars visible when 
we look toward the Galaxy than when we look in other directions 
indicate that the stellar universe is shaped somewhat like a thin 
cheese? (In answering this question, assume that the stars are 
distributed with some uniformity through the space which they 
occupy.) 

19. Might the appearance of the Galaxy be accounted for on the 
supposition that it is an irregular ring of closely packed stars sur- 



rounding us? 




Fig. 186. 



20. Spectroscopic observations show 
that a star is approaching us at the 
rate of thirty miles a second, and visual 
observations show that it is apparently 
moving perpendicular to the line of 
sight with a velocity of forty miles a 
second ; according to the principles of 
mechanics its real velocity is repre- 

Prove that 



sented by the diagonal of the rectangle in Fig. 186 
the real velocity is fifty miles a second. 



THE FIXED STARS. 255 

21. If the earth and a certain star are moving, at a given time, 
with the same velocity in the same direction, will the lines of the 
star's spectrum be shifted from their normal place? 

22. Give some reasons why the stars differ in brightness. 

23. Examine Theta Tauri with the naked eye ; if you cannot see 
it double, your vision is defective. 

24. If the orbit of a certain, binary were a perfect circle, and a 
line drawn from the observer's eye to either of the stars were oblique 
to the plane of the circle, would the orbit appear to us to be a circle 
or an ellipse? 

25. If the plane of the orbit of a binary passed through an ob- 
server's eye, would one of the stars ever occult the other, if they 
were equal? 

26. If one star of a binary is more massive than the other, to 
which one will their common centre of gravity lie the nearer? 

27. If one component of a binary is much brighter than the 
other, does it follow that it is more massive ? 

28. If the earth were fixed, and the plane of the orbit of a binary 
were perpendicular to a line drawn to the star from the observer's 
eye, would the spectroscope enable us to determine the velocity of 
either star? 

29. As binaries revolve, do the components sometimes appear 
closer together than at others? 

30. If the plane of the orbit of a binary passed through the ob- 
server's eye, how would the star appear in a telescope, when one 
body was between us and the other? 

31. What does the name Y Cygni signify? 

32. What is the signification of the designation Nova? 

33. If the outburst of a temporary star be due to the collision of 
some star with a meteor-like cloud of comparatively small bodies, 
why does it gradually fade away? 

34. Upon the collision theory how can the reappearance of Nova 
Aurigae in August, 1892, be explained? 

35. Suppose Mira to be a dense cluster of meteoric bodies, about 
which another cluster is revolving, in a very elliptical orbit. Could 
the variability of Mira be accounted for by the hypothesis of a peri- 
odic collision between Mira and its companion? 

36. Could the fact that Mira, when brightest, may be anywhere 



256 DESCRIPTIVE ASTRONOMY. 

from the second to the fifth magnitude, be explained by the collision 
theory advanced in the preceding exercise? 

37. If Y Cygni is a binary consisting of two stars of equal size 
and brightness, will its minima occur when one of the stars is behind 
the other? 

38. If the ellipses in Fig. 185 lay " endwise" to the earth, so that 
our line of vision went through the points E and F, would each in- 
terval between successive minima of Y Cygni be 36 hours? 

39. If the ellipses in Fig. 185 did not lie either exactly " broad- 
side " or " endwise " to our line of vision, would the time intervals 
between successive minima of Y Cygni be equal? 

40. Is there any reason not mentioned in § 380 why one of 
the stars would traverse the arc BDA of Fig. 185 more quickly 
than the arc AFB? 



THE NEBULA. 257 



CHAPTER XIII. 

THE NEBULA. 

" This world was once a fluid haze of light, 
Till towards the centre set the starry tides, 
And eddied into suns, that wheeling cast 
The planets." 

Tennyson. 

385. Various Forms. — Nebulae are cloud-like objects of a bewilder- 
ing variety of forms. They are to be carefully distinguished from 
clusters, which are aggregations of stars. A true nebula does not 
consist of separate stars. Many clusters, however, have nebulous 
matter associated with them, and many nebulae contain stars within 
their borders. A large nebula is in general of an irregular shape. 
In it are to be seen many spots brighter, and presumably more 
condensed, than the rest of the nebula : there are also found dark 
spots, rifts, and streams of various shapes. The cuts of the nebulae 
of Orion and Andromeda (Figs. 189 and 190) illustrate these pe- 
culiarities. 

Spiral nebulae, of which there are several, exhibit convolutions 
like those of the hair-spring of a watch ; the appearance resembles 
that of a Catherine wheel. 

Annular nebulae are ring-shaped objects, darker in the centre 
than at the edge. 

Planetary nebulae have small round disks of approximately uniform 
brightness throughout. They are usually brightest in the centre. 

A nebulous star has a strong central condensation, surrounded 
by a nebulous envelope. It is frequently difficult to decide whether 
an object should be called a planetary nebula or a nebulous star. 

Double and variable nebulae are known : no orbital revolution 
has been detected in the double nebulae : no law of variation is 
known for the variable ones. 

386. Kumber, Distance, and Grouping. — The number of known 
nebulae is about eight thousand. New ones are continually being 

17 






25« 



DESCRIPTIVE ASTRONOMY. 



discovered, especially by photography, but most of the discoveries 
are exceedingly faint and uninteresting. 

No nebula has yet revealed any parallax (§ 350). Yet the many 
and intimate associations of nebulae with stars lead to the belief that 
thev are at the same distances. 




Fig. 187. — The Pleiades: Photographed by Roberts.! 

In the Pleiades, photography has revealed the presence of a mass 
of nebulous matter surrounding four of the bright stars, and con- 
nected with another by a faint ray. The brightest star, and some 
smaller ones near it, are involved in a similar nebula. Other faint 
stars in the vicinity are connected by wisps of nebulous matter 
emanating from the vicinity of one of the bright stars. 

1 An English amateur astronomer. 



THE NEBULA. 259 

The multiple star Theta Ononis (§ 372) lies in a dark space in 
the Great Nebula of Orion, the four brighter stars looking like eggs 
in a bird's nest. The appearance suggests that the stars are con- 
densations formed from the surrounding nebulous matter. Further- 
more, certain lines in the spectrum of Theta Ononis are matched by 
corresponding ones in the spectrum of the nebula. 

While the stars are crowded together in the vicinity of the Milky 
Way, the majority of the nebulae lie outside of it. Their law of distri- 
bution over the sky is opposite to that of the stars (§ 346). They are 
most numerous where the stars are least numerous, and vice versa. 

387. Sizes : Changes of Appearance. — Nebulae vary greatly in size. 
Some are so small as to look like stars in a small telescope. Others 
are the most gigantic objects ever revealed to the eye of man. The 
Great Nebula in Orion, as recently photographed, covers a large 
part of the entire constellation. 

There are serious discrepancies between old drawings of some of 
the nebulae and recent delineations of them. Drawings of so faint 
objects, made with telescopes of different sizes and under widely 
different circumstances, would naturally fail to agree. While many 
of the apparent changes are due to such causes, there remains a 
small residuum of cases which cannot be explained reasonably, ex- 
cept on the hypothesis that real changes in the nebulae have taken 
place. The " Trifid " Nebula shown in Fig. 188 is an illustration in 
point. A star, which was located in one of the dark lanes at the 
opening of the nineteenth century, is now involved in the nebulous 
matter. The star has not changed its position with respect to the 
neighboring stars : therefore the nebula must have changed. Such 
is the result of an investigation made by Professor Holden, Director 
of the Lick Observatory. 

388. Spectra. — About half of the nebulae give spectra containing 
bright lines; thus showing (§ 73) that they may be composed of 
glowing gas under low pressure. Four of these lines are generally 
seen without difficulty with the powerful spectroscopes now em- 
ployed. Two of them demonstrate the presence of glowing hydro- 
gen. The origin of the other two is unknown. Resides these 
characteristic nebular lines, several others have been seen ; by some 
of these, helium and sodium are fairly recognized. The Great Nebula 
in Orion is the finest specimen of this class. 



26o 



DESCRIPTIVE ASTRONOMY. 



Most of those nebulae which do not exhibit bright-line spectra 
give continuous spectra (§ 73) simply. They may be composed of 
gaseous matter under high pressure, or of glowing liquid matter, or 




Fig. 



The Trifid Nebula. 



of a mixture of both. The Nebula in Andromeda belongs to this 
class : it is plentifully besprinkled with stars. Incandescent solid 
matter, unenveloped by a gas, would give a continuous spectrum. 



THE NEBULA. 26 1 

We have no proof, however, that matter exists in that form any- 
where in the universe. A few nebulae give both continuous and 
bright-line spectra. 




Fig. 189. — The Nebula in Andromeda: Photographed by Roberts. 

389. The Nebula in Andromeda. — This nebula is plainly visible to 
the naked eye, and has often been mistaken for a comet. It has a 
tolerably regular elliptical outline, and a strong central condensation. 



262 DESCRIPTIVE ASTRONOMY. 

Fig. 189 gives the impression that it is surrounded by rings like 
those of Saturn, or that it is a gigantic spiral. The appearance of 
this nebula is very interesting in its relation to the nebular hypothe- 
sis (§ 394), that all stars and planets were formed by the conden- 
sation of nebulous matter. According to this theory, the two 
condensations outside of the main elliptical portion may be planets 




Fig. 190. — The Nebula in Orion: Drawn by Bond at the 
Harvard College Observatory. 

in the process of formation. In August, 1885, a new star appeared 
close to the nucleus of the nebula ; at first it was bright enough to 
be seen with an opera-glass, but it faded away to invisibility in a few 
months. Its spectrum was almost the same as that of the nebula ; 
hence the star was probably in the nebula. It exhibited no sensible 
parallax. 



THE NEBULAE. 263 

390. The Great Nebula of Orion. — In the sword-handle of Orion 
are three stars in a line, easily seen with the naked eye. The central 
one of these appears hazy : it is the multiple star Theta Ononis, 
shown in Fig. 190, near the centre of the nebula. This star is com- 
monly known as the Trapezium, because the four brighter stars in it 
form that geometrical figure. The spectrum of one of these stars 
has been photographed, and exhibits bright lines corresponding 
to lines in the spectrum of the nebula. This indicates that the 
star is a sphere of nebulous matter, not yet condensed as much as 
stars ordinarily are. The brightest portion of the nebula is in the 
immediate vicinity of the multiple star. Thence it branches off in 
wonderful forms, which contrast beautifully, in their delicate tracery, 
with the blackness of the adjacent regions. Photography reveals a 
vast extension of the nebulosity which the most powerful telescopes 
fail to show. 

Keeler has determined spectroscopically that the nebula is retreat- 
ing from us at the rate of nearly eleven miles per second. 

If the moon be absent, the nebula, even in a small telescope, must 
call forth the admiration of the beholder. It is the finest object of 
its class in the heavens. 

391. Other Notable Nebulae. — The Dumb-bell Nebula in Vulpecula 
(between Lyra and Delphinus) appears in a small telescope to be 
composed of two oval masses in contact. 

The Ring Nebula in Lyra is situated a third of the way from Beta 
to Gamma Lyrae. It is the only one of its kind which can be seen 
with a small telescope, and is shown in Fig. 191, as seen in a 15-inch 
telescope. 

Of Spiral Nebulae, one of the most remarkable is the one in Canes 
Venatici, shown in Fig. 192. It is three degrees from the star in the 
end of the tail of the Great Bear. The appearance is as if a slow 
rotation were taking place. 

The Trifid Nebula is situated in Sagittarius : it is distinguished 
by the curious triple-pronged dark area, which gives it the appear- 
ance of being cracked open. This is the nebula previously men- 
tioned, which affords distinct evidence of change. It is shown in 
Fig. 188. 

392. Real Form of Spiral Nebulae. — While these nebulae exhibit to 
the eye, more or less perfectly, the appearance described in ^ 385, 



264 DESCRIPTIVE ASTRONOMY. 

this may not be their real form, since we see simply their projec- 
tions on the sky. In 1888 Professor Holden discovered, at the Lick 
Observatory, that one of the planetary nebulae had a spiral filament 




Fig. 191. — The Ring Nebula in Lyra: Drawn by Bond at the 
Harvard College Observatory. 

within it. This led him to a study of the best extant drawings of 
the spiral nebulae. He found that their various forms can be ex- 
plained on the assumption that the filaments which give the spiral 
appearance are really of the form of a corkscrew. He bent a wire 



THE NEBULA. 



205 



into this shape, and was able, by holding it in different positions, to 
represent the shapes of the various spirals shown in the drawings. 

393. The Magellanic Clouds. — The Magellanic Clouds, or Nube- 
cula^, are situated in the southern celestial hemisphere, and are not 
visible in middle north latitudes. They are two cloudy masses of 




Fig. 192. — The Spiral Nebula in Canes Venatici : 
Photographed by Roberts. 



light, the larger one of which has an area about equal to that of the 
bowl of the Great Dipper ; the smaller one is only one fourth as 
large. Both are plainly visible to the naked eye, and resemble por- 
tions of the Milky Way. They exhibit a marvellous structure in a 
telescope: nebulae, both regular and irregular in form, and star clus- 
ters of all degrees of condensation, are mingled promiscuously. The 
larger cloud contains about three hundred of these objects. 



266 



DESCRIPTIVE ASTRONOMY. 



THE NEBULAR HYPOTHESIS. 

394. General Statement. — The celebrated Nebular Hypothesis is 
an attempt to account for the present form of the solar system by 
a process of orderly evolution. Its name indicates that it pre- 
supposes the existence of a nebulous mass, the parent of the well 
ordered worlds which we now behold. The chaotic mass of world 
stuff may be described, in Milton's words, as 

" A dark, 
Illimitable ocean, without bound, 

Without dimension, where length, breadth, and height, 
And time and place, are lost ; where eldest Night 
And Chaos, ancestors of Nature, hold 
Eternal anarchy, amidst the noise 
Of endless wars, and by confusion stand." 

Various writers suggested, and partially worked out, the nebular 
hypothesis, but the first to give it an extensive mathematical devel- 
opment was Laplace. We proceed to state his theory, and its modi- 
fications, setting forth afterwards the facts which give color to it. 

395. Laplace's Theory. — Accord- 
ing to this theory the original neb- 
ula was a mass of intensely heated 
gas, which had by reason of the 
mutual attractions of its particles 
assumed a globular form, and had 
acquired a motion of rotation. 
As its heat was radiated away, the 
nebula contracted, and rotated more 
swiftly ; the mass became flattened 
at the poles, and when the " cen- 
trifugal force" at the equator bal- 
anced the force of gravity there, 
a ring of equatorial matter was 
abandoned. The spheroid left 
within the ring rotated still more 
rapidly until another ring was left 
behind, etc. The matter in each ring gradually condensed into a 
planet, which in turn rotated and abandoned rings, which usually 




Fig. 193. — Laplace. 



THE NEBULAE. 267 

condensed into satellites : in the case of Saturn some of the rings 
failed to break up into satellites of goodly size. 

396. Changes in the Theory. — As facts and laws unknown in La- 
place's day have been discovered, various modifications of the 
original theory have been proposed. It is no longer necessary 
to suppose that the parent nebula was originally at a high temper- 
ature : it is regarded as more probable that it was a cold cloud 
of finely divided matter, which became heated in the process of 
contraction. 

Since some parts of the parent mass were probably denser than 
others, it is not likely that rings were usually abandoned, but rather 
that balls of matter were left behind. When the material at the 
equator was unusually homogeneous, a ring similar to Saturn's 
might be formed. 

Laplace supposed that the outermost planet was formed first, but 
it is now believed that several of the planets may have been liberated 
at about the same time. 

Faye, a French astronomer, has shown that the inner planets 
may have been formed before the outer ones. 

The retrograde motions of the satellites of Neptune and Uranus 
contradict Laplace's supposition that the rings from which they were 
formed rotated as if solid, before they broke up to form the satellites. 
But if it is admitted that different portions of the ring were of differ- 
ent degrees of density, and that the inner edge rotated more swiftly 
than the outer, mathematicians find no difficulty in accounting for 
the retrograde motions of the satellites. 1 

According to Laplace's theory alone the inner satellite of Mars 
should not complete a revolution in less time than that planet re- 
quires to rotate on its axis. This anomaly has been explained in a 
marvellous manner as a result of the tides which the sun causes 
on Mars. 2 

397. The Evolution of Double Stars. — Laplace's theory of the aban- 
donment of rings, which gradually condensed into satellites, answers 
very well for the solar system, but fails for the double stars. In the 
solar system we have a number of comparatively small planets re- 

1 See Young's General Astronomy, Art. 914. 

2 An elucidation of this matter in a popular way is found in Ball's Story of the 
Heavens, Chapter XXVII. 



268 DESCRIPTIVE ASTRONOMY. 

volving about a central body, which is 750 times as massive as all its 
planetary attendants put together. But the two bodies composing 
a double star are more nearly equal to each other. If the original 
nebula were quite homogeneous, rings might be formed as supposed 
by Laplace. But as there were probably great differences in density 
in different parts of the parent nebula, the densest portions would 
attract to themselves the surrounding matter. Under such con- 
ditions it is probable that the rotating and contracting nebula would 
separate into two or more portions. 

Dr. See : has specially emphasized the fact that, while the ring 
formation is ideally possible, the nebula would be more likely to 
separate into two globular masses. Many double nebulae are known, 
which seem to substantiate this theory of " fission." Probably such 
a double nebula will condense into a double star after thousands or 
millions of years have elapsed. 

398. Testimony of the Nebulae. — We have seen that the nebulae 
are aggregations of tenuous matter, ranging from the vast filmy 
irregular nebulae to the neat round compact planetary nebulae. Be- 
tween these two extremes there seems to be every gradation of size 
and brightness. 

The great nebula of Andromeda and those which are distinctly 
spiral give the impression that they may be rotating. The globular 
bright spots found in some of the larger nebulae look as if they were 
condensations of the surrounding matter. 

Planetary nebulae usually have a brightening at the centre, and 
nebulous stars seem to be approaching the end of the process of 
transformation into stars. 

The nuclei of planetary nebulae and stars like those in the Trape- 
zium of Orion (Theta Ononis) exhibit spectra similar to those of 
the nebulae in which they are involved. Immediately around the 
stars of the Trapezium there is a dark place, as if the matter once 
there had been used up to form the stars. 

399. Testimony from the Stars. — The wonderful associations of 
nebulae and stars, such as are found in the Pleiades and in Orion, 
point to a close connection. Some of these stars have wisps of neb- 
ulous matter clinging to them, as photography has shown. Others, 

1 Dr. T. J. J. See, Professor in the University of Chicago, who has worked out an 
elaborate theory of the evolution of double stars. 



THE NEBULA. 269 

though giving the ordinary spectra of stars (§ 353), have quite an 
extensive nebulosity connected with them. The Wolf-Rayet stars 
give bright line spectra, and one class of the nebulae does. One 
naturally concludes that we have different stages of a process of con- 
densation, which will finally lead to the production of such highly 
finished orbs as Sirius, or the sun. 

400. Testimony of the Earth and Moon. — The deeper we go into 
the crust of the earth, the warmer we find it. Volcanoes give abun- 
dant evidence of the presence of intensely heated matter in the in- 
terior of the earth. The granite which we prize so highly owes its 
toughness to its having passed through primeval fires. Statuary 
marble is but common limestone which has been metamorphosed by 
heat Mountain chains are thought to have been formed by the 
wrinkling of the earth's crust, as it contracted in the process of cool- 
ing. The earth and the sun are composed of the same substances in 
large part. Were the former heated to incandescence it would give 
essentially the same spectrum as the latter. 

The moon bears the marks of its igneous origin, written in large 
characters over its face. The following extract is taken from 
Nasmyth and Carpenter's book on the Moon :— 

" We trust that we, on our part, have shown that the study of the moon 
may be a benefit not merely to the astronomer, but to the geologist, for we 
behold in it a mighty * medal of creation,' doubtless formed of the same ma- 
terial and struck with the same die that moulded our earth ; but while the 
dust of countless ages and the action of powerful disintegrating and denud- 
ing elements have eroded and obliterated the earthly impression, the super- 
scriptions on the lunar surface have remained with their pristine clearness 
unsullied, every vestige sharp and bright as when it left the Almighty 
Maker's hands." 

401. Testimony from the Planetary Systems. — We note the following 
harmonies in the motions and densities of the planets. 

I. They all revolve eastward about the sun, in orbits nearly cir- 
cular, which lie approximately in the same plane. 

II. They rotate eastward on their axes (except probably Uranus 
and Neptune), the planes of their equators being but little inclined to 
those of their orbits (except probably that of Uranus). 

III. The satellites revolve in the same direction in which the 



27O DESCRIPTIVE ASTRONOMY. 

planets rotate, their orbit planes being nearly coincident with the 
equators of their respective planets. 

IV. The four inner major planets are small bodies of great 
density ; they rotate slowly, as far as is known. The four outer 
major planets are great bodies of small density ; they rotate swiftly, 
as far as is known. 

402. Testimony of the Sun. — The accepted theory of the source 
of the tremendous quantity of heat continually radiated by the sun 
is the contraction theory (§ 86). If the sun be now contracting 
it must have been larger 1,000 years ago than to-day. Reasoning 
backward, we find it highly probable that at one time the diameter 
of the sun equalled that of the orbit of Mercury. But we may go 
yet farther back in imagination and see the sun as a tenuous nebu- 
lous mass, the confines of which lie beyond the orbit of the farthest 
planet. 

403. Is the Testimony Sufficient? — The human mind is irresistibly 
attracted toward a grand and far-reaching theory, which explains a 
variety of observed results by a single process of development. 
With a limitless duration of time and an infinite extent of space at 
its disposal, it leaps over the most stupendous chasms in knowledge 
as nimbly as a mountain goat leaps from rock to rock, scaling the 
precipitous heights of its native wilds. 

The limitations of our knowledge are so great that the Nebular 
Hypothesis must probably remain a mere theory, as long as man in- 
habits the earth ; Bacon has said that the subtlety of nature tran- 
scends in many ways the subtlety of the intellect and senses of man. 
Yet the theory explains many facts of observation so simply and so 
reasonably, that the speculations of men will probably be guided by 
its broad lines for centuries to come. 

So inadequate is the sum total of our present knowledge of the 
processes of celestial evolution that we are led to cry out with Job : 
•' Lo, these are parts of His ways : but how little a portion is heard 
of Him? but the thunder of His power who can understand? " 

404. The Future of the Visible Universe. — As the sun is continually 
radiating its heat away, with boundless prodigality, it is reasonable 
to suppose that the stars, which are but distant suns, are doing like- 
wise. We know of no way in which this expenditure is to be repaid. 
We can look forward to the time when the sun will become a cold 



THE NEBULAE. 27 I 

cinder, feeling its way by the starlight through the darkness of infi- 
nite space. But will there be starlight then? Many of the stars are 
larger and hotter than the sun, and, though much diminished in radi- 
ance, will yet be able to shed a kindly though feeble light upon his 
pathway. But the time will come when even the brightest and hot- 
test, having radiated its heat away, will roll a cold corse among its 
dead compeers. Such is the gloomy teaching of our philosophy. 

Once there lived a race of ephemerans, whose dwelling place was 
upon a thermometer. The span of life of one of them was but a 
second. Being of a scientific turn of mind they made records of the 
readings of the instrument. After observations had been made for 
ten generations they promulgated the theory that the mercury was 
rising one hundredth of a degree every second. After the lapse of 
ten generations more the theory was confirmed, and was then called 
a law. When one hundred generations had passed away, the law 
was considered so firmly established, that no reasonable ephemeran 
could doubt it. It was the one grand and inexorable law of nature : 
one might question everything else, but never this. During the next 
ten generations they executed a laborious triangulation, determin- 
ing the distance over which the mercury must still travel before it 
reached the top of the thermometer, and burst the glass tube. Then 
it was an easy matter to calculate that the utter ruin of their beauti- 
ful dwelling place could not be delayed beyond the ten-thousandth 
generation. 

Great was the humiliation of their scientists, but still greater the 
joy of the ephemerans at large, when it was found, after the lapse of 
two thousand generations, that the mercury was actually going the 
other way. Even the scientists were constrained to admit that there 
were more things in heaven and earth than were dreamed of in their 
philosophy. 



272 DESCRIPTIVE ASTRONOMY. 



CHAPTER XIV. 

THE CONSTELLATIONS IN DETAIL. 

" Sit, Jessica. Look how the floor of heaven 
Is thick inlaid with patines of bright gold : 
There 's not the smallest orb which thou behold'st 
But in his motion like an angel sings, 
Still quiring to the young-eyed cherubins." 

Shakespeare. 

405. The Greek Alphabet. — Since very many of the stars are 
named by means of Greek letters, the Greek alphabet is subjoined. 
In pronouncing the names of the letters e should be given like ay 
in bay : 1 is pronounced like ee. 



a 


Alpha. 


ft 


Beta. 


y 


Gamma. 


S 


Delta. 


e 


Epsilon. 


t 


Zeta. 


7] 


Eta. 





Theta. 


i 


Io'ta. 


K 


Kappa. 


A 


Lambda. 


fl 


Mu. 



V 


Nu. 


£ 


XI (Ksee) 





Omicron. 


7T 


PL 


P 


Rho. 


a 


Sigma. 


T 


Tau. 


V 


Upsilon. 


<t> 


Phi. 


X 


Chi. 


* 


Psi. 


(O 


OraeVa. 



406. Use of the Data in this Chapter. — Before making use of the 
data in this chapter, the student should be familiar with §§ 1 and 8-12. 

Under each constellation the directions for finding its principal 
stars are first given. These directions presuppose an acquaint- 
ance with Ursa Major, Ursa Minor, and Cassiopeia (§ 10). They 
should be used in conjunction with the maps. 

When attention has been called to the configuration of the princi- 
pal stars of the constellation, the chief objects of interest in it are 



THE CONSTELLATIONS IN DETAIL. 273 

mentioned. The numbers in [ ] refer to the lists at the end of the 
chapter. For the mythological history of the constellations, the 
reader is referred to a classical dictionary. 

At the end of the chapter the right ascensions and declinations 
of interesting telescopic objects are given, together with simple 
directions for finding them by means of a telescope equatorially 
mounted, and provided with graduated circles. 

407. Andromeda, the Chained Lady. [Map I.] — Andromeda may 
be easily learned after Cassiopeia. A line drawn from Polaris to 
/3 Cassiopeiae. and prolonged an equal distance, strikes a, which is the 
head of Andromeda : it is also one corner of the square of Pegasus. 
A line drawn from Polaris to e Cassiopeiae, and prolonged nearly the 
same distance beyond, ends very near 7, which is in one foot of the 
figure. The row of stars from a to 7 bounds the left side of An- 
dromeda, so that her body lies between this row and Cassiopeia. 
Her outstretched arms run from A to 77. 

The great nebula [83] which is plainly visible to the naked eye 
is in line with /3 and yet, jjl being half way between the other two. 

It has been described in § 389. In a small telescope it is simply 
a bright oval mass, none of the wonderful details of its structure 
being perceptible. 

7 [6] is a fine double, the larger star being orange and the 
smaller sea-green : the small star is a very close double ; 7 is there- 
fore really a triple star. The small star is of the sixth magnitude, 
and 1 \" distant. 

408. Aquarius, the Water Bearer. [Map V.] — A line drawn from 
/3 Pegasi to a of the same constellation, and prolonged as far again, 
terminates just east of a group of fourth magnitude stars having the 
form of a Y. This is the jar from which Aquarius pours a never 
exhausted stream of water which meanders southward into the 
mouth of the Southern Fish. 

The remainder of this dull-looking constellation lies south of the 
jar, and extends to quite a distance east and west of it. Thirty 
degrees south and a little east of the Y is the first magnitude star 
Fomalhaut, in the Southern Fish. Between the two lies a portion 
of Aquarius which has been likened to the contour of South Amer- 
ica. It is formed by the stars 0, A, t, 8, c 2 , and t. The stream from 
the water jar to the Southern Fish is marked by pretty groups of 



274 DESCRIPTIVE ASTRONOMY. 

stars, and is indicated on the map by a dotted line. The stars a, /?, 
v, e, and 3, in the western portion of the constellation, form a rude 
short-handled dipper. 

f [81], the central star of the Y is a fine binary, the components 
being nearly equal ; they are 3" apart now. 

A little over a degree west of v lies a small bright planetary neb- 
ula [136], with a stellar nucleus: it is of a greenish blue cast. 

Almost directly north of /3, at a distance of 5 , lies an exceedingly 
compact globular cluster [138] of faint stars. 

409. Aquila, the Eagle. [Map V.] — Altair, the brightest star, is 
easily found by means of /3 and 7, which lie on either side of it; the 
three stars lie athwart the Milky Way, there being no other very 
bright stars in the immediate vicinity. The triangle formed by 7, 
0, and X embraces most of the bright stars of the constellation, 
which bears no resemblance to an eagle. 

A degree and a half northeast of 7 lies the sixth magnitude star 
7r [69], which is a close double, a test for the power of a three- or 
four-inch telescope, on a fine night; the components are nearly 
equal, and only i".5 apart. 

Three degrees east and a trifle south of 12 Aquilse lies a fan- 
shaped cluster [131] of telescopic stars, t] varies between the 
fourth and fifth magnitudes in a period of seven days and a fraction. 

410. Argo Navis, the Ship. [Map III.] — Only a portion of this 
huge constellation is visible in the United States. The rest is too 
far south. The few bright stars visible to us lie east and south of 
Canis Major, and can be identified by the use of the map, after that 
constellation is known. 

A line from Sirius to 7 Canis Majoris, when prolonged nearly 
twice as far, terminates just north of a diffuse cluster [102] of stars, 
some of which are visible to the naked eye. 

A little over a degree east of the preceding and 20' south of it is 
a circular telescopic cluster [103] 30' in diameter. A degree west 
of [102] lies a red star [150] of the sixth magnitude. 

411. Aries, the Ram. [Map II.] — A line from Polaris through 
e Cassiopeiae to 7 Andromedae, when prolonged a distance equal 
to that between the latter stars, pierces the triangle composed 
of a, ft, and 7 Arietis, which is the distinguishing mark of the 
constellation. The triangle is in the Ram's head. His body lies 



THE CONSTELLATIONS IN DETAIL. 275 

to the eastward, and bears no resemblance to the configuration of 
the stars. 

7 [4] is a fine double star, the components of which are nearly 
equal ; the distance between them is 9". 

412. Auriga, the Charioteer. [Map I.] — Capella, the brightest star, 
is of the first magnitude, and forms a rude square with Polaris, e Cas- 
siopeia^, and o Ursas Majoris, the star in the nose of the Great Bear. 
It is one of the brightest stars in the sky, and shines with a pure 
white light, ft, 6, 1, and ft Tauri form with it an irregular five-sided 
figure, which is readily discerned. 8 is in the head of the Charioteer : 
1 and ft Tauri mark his feet. He carries in his arms a kid marked 
by the stars e, f, and rj. 

14 Aurigae [14] is a triple star having a seventh magnitude com- 
panion at a distance of 14" and one of the eleventh magnitude at 
a distance of 13". 

Inside of the triangle formed by X, 1, and % are a number of star 
clusters most of which lie near the line between X and %. Half way 
between X and t is a rich field of stars [90] fainter than the seventh 
magnitude. A line from ft to a point midway between 6 and v, pro- 
longed as far again, strikes a beautiful cluster [96] of small stars : 
the whole field seems strewn with gold dust. These stars are so 
closely associated that one must believe them to be really near to- 
gether, and not merely in the same line of vision. This combina- 
tion of stars of very different degrees of brightness is an evidence 
that a faint star is not necessarily at a great distance from us. A 
line. from 1 to 0, prolonged half its length, ends near a deep red 
star [149] of the sixth magnitude. 

413. Bootes, the Bear Keeper. [Maps I. and IV.] — A line from 
8 Ursae Majoris to rj of the same constellation, prolonged an equal 
distance, strikes a very small triangle composed of the fourth mag- 
nitude stars 0, 1, and k, which are in the uplifted hand of Bootes. 
They lie midway between Polaris and Arcturus, the most brilliant 
star in Bootes, and one of the brightest in the heavens : it has a 
pronounced ruddy hue. It is at the lower end of an immense kite- 
shaped figure formed by /3, 8, e, a, p, and 7. ft is in the head of 
Bootes; 7 and 8 are in his shoulders; p and e form his belt. In his 
right foot is the triangle f, 0, it; in his left foot is another triangle. 
rj, t, v. Arcturus is in his sword. 



276 DESCRIPTIVE ASTRONOMY. 

e [41] is a fine slow binary : the companion is of the sixth magni- 
tude, and 3" away. The large star is yellow, the small one blue. 

f [42] is a fine binary, having a period of about 130 years. The 
companion is now less than 4" distant, of the seventh magnitude, 
and purple. The distance is diminishing. 

1 [39] i s 5° east an d 2 ° north of 77 Ursae Majoris, and has a 
companion of the eighth magnitude, 38" away. 

it [40] is 6° east and 3 south of Arcturus, and has a companion 
of the sixth magnitude, 6" distant. 

414. Camelopardus, the Camelopard. [Map I.] — The stars in this 
constellation are faint. The head of the creature consists of four 
fifth magnitude stars situated one fifth of the way from Polaris to 
the bowl of the Great Dipper. His fore feet are almost on the 
head of Auriga, while his hind feet are in position to give Perseus 
a kick in the stomach. If he were not such a weakling, he might 
give trouble. 

There is a rich, though coarse cluster [89] two thirds of the 
way from a Persei to 8 Aurigse. It is close to the fifth magnitude 
star 7 Camelopardi. 

415. Cancer, the Crab. [Maps III. and I.] — The principal start 
form an inverted Y, as shown on Map III. A line drawn from 
Polaris to h Ursae Majoris, a fifth magnitude star in the head of 
the Great Bear, and prolonged 1^ times its own length, strikes 1, the 
uppermost star in the A. 

f [27] is found by alignment with Castor and Pollux. It is a 
fine multiple star, and has been described in § 372. The two stars 
forming the bright one are in rather rapid motion, sixty years 
sufficing for a revolution. The visible companion of this binary is 
5" distant. The large star looks oblong with a high power on a 
three- or four-inch telescope. 

Between 7 and 8 lies the cluster [104] Praesepe, the Beehive, 
which is visible to the naked eye on a moonless night. A good- 
sized opera-glass shows it better than a larger telescope. 

416. Canes Venatici, the Hunting Dogs. [Map I.] — This con- 
stellation is not especially noteworthy. Its brightest star, a or 12, 
is called Cor Caroli (the Heart of Charles II. of England), and is 
found by prolonging a line from Polaris to e Ursse Majoris half its 
length. 



THE CONSTELLATIONS IN DETAIL. 277 

Cor Caroli [36] has a sixth magnitude companion 20" distant. 

2 [33] is an orange star of the fifth magnitude, having a blue 
companion of the ninth magnitude, 11" distant. 

There is a bright globular cluster [114] containing upwards of 
1,000 stars, lying nearly midway between Cor Caroli and a Bootis 
(Arcturus), but a little nearer the latter: it is close to a star of the 
sixth magnitude. 

The Great Spiral Nebula [113] (§ 391) lies about one fourth of 
the way from rj Ursae Majoris to Cor Caroli. It is called the Whirl- 
pool Nebula, but in small telescopes it looks simply like a faint 
double nebula. The entire constellation is plentifully besprinkled 
with faint nebulae. 

417. Canis Major, the Great Dog. [Map III.]- — This constella- 
tion is best learned after Orion. The line of the three stars in 
Orion's belt prolonged eastward passes near Sirius, which is a 
in this constellation, and by far the brightest fixed star in the 
heavens. The triangle 6, e, 77 is in the haunches of the animal, and 
Sirius is in his head : j3 is the extremity of one uplifted fore paw. 
The animal sits upright, in the attitude of begging his master 
Orion for permission to put his teeth into the Hare, which is under 
Orion's feet. 

Sirius is a very interesting double star (§ 370), but is much too 
difficult for a small telescope, the faint companion being in the 
blaze of light surrounding the bright star. The period of revolution 
is about fifty years. 

fjb [24] has a ninth magnitude companion at a distance of less 
than 4". 

A superb cluster [100] visible to the naked eye lies about one 
third of the way from Sirius to e. There is a ruddy star near the 
centre : many of the brighter stars are arranged in curves. 

418. Canis Minor, the Little Dog. [Map III.] — It is well to learn 
Gemini before Canis Minor. A line from Polaris to /3 Geminorum 
(Pollux), prolonged one third as far again, reaches Procyon, a first 
magnitude star, which is a Canis Minoris. The only other con- 
spicuous star is /3, which is 4 northwest of Procyon. 

Procyon is of interest because of its irregular proper motion, 
supposed to be caused by the presence of close companions, which 
have often been searched for by the largest telescopes, but without 



278 DESCRIPTIVE ASTRONOMY. 

success. In the same field with Procyon is a star of the seventh 
magnitude which is a close double. The components are only 1".$ 
apart, but the star can be elongated by a good four-inch glass. 

419 Capricornus, the Goat. [Map V.] — It is well to know Cyg- 
nus before attempting to learn Capricornus. A line from Polaris 
to 7 Cygni (where the arm of the cross is fixed to the upright 
piece), prolonged an equal distance, reaches the naked-eye double 
a, below which is ft at a distance of 2°. 5. c and 7 form another 
such pair of stars, and ^r and 00 a third. The constellation 
is chiefly embraced in the triangle formed by these three pairs. 
a and (3 are in the head of the animal, while yjr and 00 are in his 
knees ; the rest of the Goat may be supplied to suit the fancy. 

p [73] and 77 [72] are pretty doubles, each having a companion 
of the ninth magnitude, less than 4'' away. A good night is needed 
for their observation with a four-inch glass. 

420. Cassiopeia, the Lady in the Chair. [Map I.] — This brilliant 
constellation is quickly found by using Map I. according to the 
directions in § 10. The stars e, 8, 7, a, /3, and k form a rude broken- 
backed chair. The Lady, however, refuses to sit in it, preferring 
to sit on empty space. The stars /3, a, 7, and tc form her body; 
8 is in her knee, and t in her foot ; f is in her head ; her arms 
are uplifted, possibly in prayer to the gods to spare her lovely 
daughter Andromeda, who has been chained to a rock, as prey for 
a sea monster. 

r] [1] is a splendid binary, having a purple companion of the 
eighth magnitude, 5" distant. It is less than half the way from 
a to 7. The period of revolution is 200 years : the combined mass 
of the two stars is thought to be from five to ten times that of the 
sun. Near /c appeared Tycho's new star described in § 375. 

As the Milky Way runs through Cassiopeia, there are many 
beautiful fields which can be best seen with a low power. 

Near /3, between p and <r, is a large cloud of minute stars 
[142] discovered by Caroline Herschel, the sister of Sir William 
Herschel. 

Between it and in the uplifted left hand of the Lady is a 
magnificent region. 

One degree east and a little north of 8 is a beautiful field [84]. 
A line from 8 to a, prolonged 1^ times its former length, ends near 



THE CONSTELLATIONS IN DETAIL. 279 

R, a vivid red star which varies from the fifth to the twelfth magni- 
tude in a period of 433 days. 

421. Centaurus, the Centaur. [Maps III. and IV.] — Centaurus, even 
when most favorably situated, is too near the southern horizon for 
satisfactory observation in the United States, except in Florida and 
Southern Texas. It is of especial interest, because it contains our 
nearest neighbor among the stars, a Centauri. 

422. Cepheus. [Map I.] — This constellation lies between Cassi- 
opeia and Draco. Cepheus is the husband of Cassiopeia, who, with 
her daughter Andromeda, nearly monopolizes the brilliancy of the 
family. The five brightest stars are a, /3, 7, 1, and f, which form a 
figure composed of a rude square surmounted by a triangle which 
is nearly isosceles, a forms with Polaris and 7 Cassiopeise an isos- 
celes triangle which is nearly equilateral. Near f are 8 and e : the 
three are in the head of the figure. 

ft \j7~\ * s a double star, the companion being blue, of the eighth 
magnitude, and 14" distant. 

8 [82] has a companion of the seventh magnitude, 41" distant: 
the primary is yellow, the companion blue : the main star is a noted 
variable, having a period of 5^- days. 

f [80] has a blue seventh magnitude companion, 6" distant. 

423. Cetus, the Whale. [Maps II. and V.] —A line from Polaris to 
8 Cassiopeiae, prolonged so that the prolongation is 2J times the ori- 
ginal length of the line, reaches the centre of this huge and ungainly 
constellation, which can be best learned by following the dotted lines 
given on the map. The monster has about the shape of a walrus. 
The most noticeable portion of the constellation is an irregular pen- 
tagon, rudely kite-shaped, formed from the third magnitude stars ft, rj, 
1 , f, and t. The pentagon formed by a, 7, £ 2 , fi, and \ marks the 
head. 

Nearly half way from 7 to flies (Mira) [143], the wonderful 
variable described in § 376. It is visible to the naked eye only six 
weeks in the year. 

7 [8] is not a very difficult double for a four-inch glass. A star 
of the seventh magnitude nestles close to the larger star : the dis- 
tance is 2 n .$. 

a [144] is a fine orange-colored star, having a blue neighbor in 
the same low-power field. 



280 DESCRIPTIVE ASTRONOMY. 

424. Columba, the Dove. [Map II.] — The full name is Columba 
Noachi or Noah's Dove. The asterism lies south of Lepus, and is 
too low down in the south to be seen well. A line drawn from j3 
Orionis (Rigel) to /3 Leporis, and prolonged as far again, terminates 
near a and ft, the two brightest stars. 

425. Coma Berenices, the Hair of Berenice. [Map I.] — This con- 
stellation consists of faint stars ; most of those visible to the naked 
eye are of the fifth and sixth magnitudes. They are well crowded 
together. A line from Polaris to S Ursae Majoris, when prolonged an 
equal distance, terminates near the most crowded part of the asterism. 
It is a fine sight in a small opera-glass. 

A little over one third of the way from rj Bootis (near Arcturus) 
to /3 Leonis is the fifth magnitude star 42 ; 50' northeast of this is a 
condensed mass of minute stars [112], which cannot be well seen 
with a telescope of less than four inches aperture. 

426. Corona Borealis, the Northern Crown. [Map I.] — Corona lies 
a little south of a line from a Bootis (Arcturus) to a Lyrae (Vega), 
at about one third the distance from the former to the latter. Seven 
of its principal stars form a figure so similar to a crown that it is in- 
stantly recognized. 

f [44] has a bluish green companion of the sixth magnitude 6" 
distant. 

A degree south of e is a ninth magnitude star called T Coronae 
[155]. It suddenly blazed up in May, 1866, and equalled a in 
brightness ; it then slowly declined, and after a month reached its 
former low estate, which it has held ever since. 

427. Corvus, the Crow. [Map IV.] — A line from Polaris to 8 Ursae 
Majoris, prolonged until it is 3J times its former length, strikes a 
small but conspicuous quadrilateral, I5°west and io° south of aVir- 
ginis (Spica). a is in the Crow's bill; the Crow stands upon and 
pecks at Hydra. 

S [34] is accompanied by a purple star of the eighth magnitude, 
at a distance of 24". 

428. Crater, the Cup. [Map III.] — Crater adjoins Corvus on the 
west, and stands upon Hydra. The stars rj, f, 7, 8, e, and 6 form the 
bowl of a crooked goblet, in the base of which are a and /3. The gob- 
let leans as if to discharge its contents upon its neighbor, the Crow. 

Just east of a and in the same field of view with a very low power 



THE CONSTELLATIONS IN DETAIL. 28 1 

is R [152], a notable red star of the eighth magnitude. Sir William 
Herschel described it as " scarlet, almost blood-colored ; a most 
intense and curious color." 

429. Cygnus, the Swan. [Map I.] — Cygnus is readily discovered 
by following the directions for using Map I. given in § 10. It lies in 
the Milky Way, just east of Lyra, and is quickly recognized by the 
cross, the upright piece of which is composed of a, 7, 77, ^, and /3, 
and has the same trend as the Milky Way. The cross arm consists 
of the stars 8, 7, and e. 

/3 is in the Swan's head, and a in its tail. The cross piece of the 
cross, extended, forms the wings of the bird. 

/3 [66~\ has a blue seventh magnitude companion at a distance 
of 34". It is the finest colored double for a small telescope in the 
northern sky ; the colors are beautifully seen by putting the telescope 
slightly out of focus. 

\i [78] is a much closer double, the fifth magnitude companion 
being only 4" distant. A third star of the seventh magnitude is 
over 200'' distant. 

17 [68] lies in a beautiful field, and has a ninth magnitude com- 
panion 26" distant. 

61 \_76~], which is in one corner of a parallelogram formed by a, 
7, e, and itself, is a pretty double when seen with a low power : the 
components are nearly equal. This star is celebrated as the first 
one the distance of which from us was measured. It is about 
550,000 times as far off as the sun. 

There are fine fields in many places, especially within a few de- 
grees of a (Deneb). One of the best is a little north of the middle 
of a line from a and 8, near o. In the northeast corner of the con- 
stellation, about half way between p and ir 1 1 is a large cluster [139] 
in a rich vicinity. 

430. Delphinus, the Dolphin. [Map V.] — A line from Polaris to 
a Cygni, when prolonged until it is two thirds longer than before, 
strikes a small diamond, composed of three stars of the fourth mag- 
nitude and one of the third. These, with a fifth of the fourth mag- 
nitude, which lies southwest of them, form a narrow wedge, called 
Job's Coffin. This is the principal portion of Delphinus. 

7 [74] is a golden yellow star having a greenish blue companion 
of the sixth magnitude, at a distance of 1 1". 






282 DESCRIPTIVE ASTRONOMY. 

431. Draco, the Dragon. [Map I.] — The head of the Dragon 
consists of a bright quadrilateral formed of /3, 7, £. and v, which is so 
situated as to form an equilateral triangle with Cassiopeia and the 
bowl of the Great Dipper, Polaris being inside of the triangle. It 
also forms a much smaller right triangle with a Lyrae (Vega) and 
a Cygni, the right angle being at Vega. 

From the head the constellation winds in magnificent convolu- 
tions, shown by the dotted line on the map, around between the two 
Bears. X, the last bright star in the tail, is two thirds of the way 
from Polaris to the centre of the bowl of the Great Dipper. 

About half way between f Ursae Majoris (Mizar) and 7 Ursae 
Minoris (one of the two brighter stars in the Little Dipper) lies a, 
which is distinguished as having been the pole star four or five 
thousand years ago. About half way between 8 and f lies the pole 
of the ecliptic, which is near a bright planetary nebula [125], 35" in 
diameter. Unlike most such objects, it can be seen very well with a 
four-inch glass. 

/jl [52] is a neat double, the two stars being nearly equal in bright- 
ness, and less than 3" apart. A planetary nebula has been men- 
tioned above. 

432. Equuleus, the Little Horse. [Map V.] — a lies 7 west and 
nearly 5 south of e Pegasi, which is in the nose of the animal. It 
contains only five stars above the sixth magnitude. 

e [75] has a companion of the seventh magnitude at a distance of 
11": the main star is a close rapid binary, which now looks elon- 
gated in a four-inch telescope, armed with a high power. 

433. Eridanus, the River. [Map II.] — Three degrees north and 
two west of j3 Orionis lies ,6 Eridani, which may be considered as 
the source of the river. Thence it flows west, following the sinuous 
line on the map, till it reaches the star ir Ceti, where it laves the 
paws of Cetus ; then it drops south about 5 , thence east, southeast, 
and southwest in succession, till it is lost beneath our horizon. 

32 [11], which has a right ascension of 3 h. 49 m. and a south 
declination of 3° 15', is a fifth magnitude star having a companion of 
the seventh magnitude 7" distant. The primary has been called 
topaz-yellow, and the companion sea-green. 

434. Gemini, the Twins. [Maps I. and II.] — A line drawn from 
the bowl of the Little Dipper to the head of the Great Bear, and 



THE CONSTELLATIONS IN DETAIL. 283 

prolonged an equal distance, terminates near the two bright stars a 
and ft (Castor and Pollux). Pollux is the brighter of the two. 
These two are in the heads of the twins, who stand side by side. 
The chief stars can be traced by the dotted lines on Maps I. and II. 
The entire figure is much like an end view of an upright piano. 
a and /3 are at the top, /x, 7, and f at the bottom, while A, and f are 
at the key-board. The summer solstice is close to the fifth magni- 
tude star 1, which is a little west and north of rj and /jl. 

Castor [26] is a magnificent double, the components differing one 
magnitude in brightness, and being nearly 6" apart. It is a binary, 
the period of which is thought to be about 1,000 years. 

8 [25] has an eighth magnitude companion at a distance of f. 

Four degrees west and two north of /jl (at the base of the back of 
the piano) is a cluster [97], visible to the naked eye as a faint cloud 
on the sky. It is 20' in diameter and consists of stars from the ninth 
magnitude down to the faintest points of light. 

435. Hercules. [Maps I. and IV.] — Directly east of Corona lies 
the belt of Hercules, composed of the stars e and f; /3 and 8 are in 
the shoulders; 77 and it are in the thighs; a marks the head. The 
limbs and arms are traced by the dotted lines on the maps. The 
whole forms a fair picture of a giant, with his head toward the 
equator. 

a [54] is a fine double, having an emerald companion of the sixth 
magnitude 5" away : it is also variable. 

P [57] is a binary, having a greenish companion of the fifth mag- 
nitude at a distance of 4" : it is near it in one thigh. 

8 [55] in one shoulder has an eighth magnitude companion, which 
has, if one compares the estimates of different observers, nearly all 
the colors of the rainbow, and is at a distance of 19". 

The finest globular cluster [118] in the northern hemisphere, 
pictured in Fig. 173, is one third of the way from r\ to f, and is just 
visible to the naked eye. The stars are so thickly crowded near 
the centre, that a small telescope shows them simply as a neb- 
ulous mass. 

About one third the way from t, in one foot, to 77, in the opposite 
thigh, is a very condensed cluster [121], which is fine, but inferior in 
interest to the preceding. 

436. Hydra, the Snake. [Maps II. and III.] —A line from Polaris 



284 DESCRIPTIVE ASTRONOMY. 

through the middle of the triangle which forms the head of the 
Great Bear, carried on through Cancer, meets the head of Hydra, 
which is just beyond Cancer ; the head is a good representation of 
that of a hissing snake. Thence it may be traced in a south and 
east direction by following the dotted line on the map. A line from 
Polaris through h at the vertex of the obtuse angle of the triangle in 
the Great Bear's head, passing in front of the Sickle in Leo (through 
k Leonis) meets a, which is also called Cor Hydrae. The distance 
from a to k Leonis is one half the distance of the latter from Polaris. 
One is helped in tracing the eastern end of the constellation by the 
recognition of Corvus, which stands upon it. 

e [28], the northernmost star in the head, has a blue companion 
of the eighth magnitude at a distance of 3". 5. 

At a right ascension of 10 h. 20 m., 2° south of ^, is a bright plan- 
etary nebula [108], which appears as large as Jupiter when the latter 
is at opposition. 

437. Lacerta, the Lizard. [Map I.] — Lacerta lies between Cyg- 
nus and Andromeda. The middle point of a line connecting a 
Cygni with a Andromedae lies a little south of the centre of the 
constellation. 

Two and a half degrees west of 7, which is the brightest star in 
the constellation, lies a fair cluster [141]. The constellation furnishes 
some fine fields, when viewed with a low power. 

438. Leo, the Lion. [Map III.]— A line from Polaris to the 
middle point of a line connecting a Ursae Majoris and h of the same 
constellation, when prolonged to nearly three times its original 
length, passes through a conspicuous figure known as The Sickle, 
and terminates at a (Regulus), in the end of the handle of the 
Sickle. At the east of this figure is a conspicuous right-angled tri- 
angle which lies in a line drawn from Polaris through the bowl of 
the Great Dipper. The Sickle constitutes the head and the fore 
part of the body of the crouching lion. The large triangle is in his 
haunches. Regulus is sometimes called The Lion's Heart. 

7 [30] is a golden yellow star, having a companion of the fourth 
magnitude, at a distance of 3". 5. It is one of the finest binaries in 
the northern sky : its period is about 400 years. 

1 [32], the nearest bright star south of the west end of the right 
triangle, has a bluish companion of the seventh magnitude, less than 
3" away. 



THE CONSTELLATIONS IN DETAIL. 285 

A little over half the way from a to f is the crimson variable R 
£151], which ranges between the fifth and tenth magnitudes ; the 
period is 312 days. 

439. Leo Minor, the Little Lion. [Map I.] — Adjoining the Sickle, 
in a line from it to the bowl of the Great Dipper, lies Leo Minor, a 
shapeless constellation containing a few naked-eye stars, three of 
which are as bright as the fourth magnitude. 

440. Lepus, the Hare. [Map II.] — Lepus crouches under Orion's' 
feet, and does not particularly resemble a hare. 

7 [21] is a triple star; the larger companion is of the seventh 
magnitude, and is 93" distant; the small companion is 45" from 
the other one, and is visible with a three-inch glass. 

45 [19] is a seventh magnitude star 1 J° east of a ; it has four com- 
panions visible with a small telescope, at distances varying from 60" 
to 126". There are four other companions to be seen with larger 
telescopes. 

A line from a to /*, prolonged two thirds of its length, ends close 
to the crimson star R [147], which varies from the sixth to the ninth 
magnitude; the period is 438 days. 

441. Libra, the Scales. [Map IV.] — Libra is best learned after 
Virgo and Scorpio, between which it lies, a lies a little more than 
half way from a Virginis ( Spica ) to /3 Scorpii. The chief config- 
uration is a quadrilateral formed by a, /3, 7, and t. 

a looks elongated to a keen eye ; an opera-glass shows that it 
has a fifth magnitude companion. 

8 [153] is a variable, situated 4°.5 west and i° north of /3. Its 
period is 2-J days, and it varies from the fifth to the sixth magnitude. 
The change in brightness consumes 12 hours. 

/3 [154] is a pale green star. 

442. Lupus, the Wolf. [Map IV.] — Lupus lies south of Libra, 
and even when best seen is too near the southern horizon for observ- 
ers in middle north latitudes. 

443. Lynx, the Lynx. [Map I.] — The Lynx occupies a dull 
region between Ursa Major on one side, and Auriga and Gemini on 
the other. The leading stars form an irregular line, traced on the 
map. 

38 [29] in the southeastern corner of the constellation, has a lilac 
companion of the seventh magnitude, 3" distant. The pair 38 and 



286 DESCRIPTIVE ASTRONOMY. 

40 form an equilateral triangle with two pairs in the feet of the 
Great Bear. 

5 [148] is a fiery red star of the sixth magnitude, in a fine group. 

444. Lyra, the Harp. [Map I.] — The leader of this constellation 
(Vega) is one of the brightest of the first magnitude stars. To the 
naked eye its color is pale sapphire. It is easily identified by means 
of the two fourth magnitude stars, e and c, which form with it an 
equilateral triangle, each side of which is nearly 2° in length. The 
constellation lies between Hercules and Cygnus. The equilateral 
triangle is perched on one corner of a rhomboid, f being common 
to both figures. 

a (Vega) [60] has a blue companion of the tenth magnitude, 48" 
distant. 

/3 [63] is a multiple star, having three companions of about the 
eighth magnitude, at distances of 46", 66", and S6'\ respectively. It 
is also one of the noted variables. See J 378. 

e [61] is one of the equilateral triangle, and appears elongated to 
the average eye : a sharp eye splits it into two stars. An opera- 
glass separates them widely, and a small telescope shows each star 
as a double. The distance between the components of one pair is 
3" ; the other pair is a little closer. 

f _62\ has a fifth magnitude companion, 44" distant. 

8° east of Vega are the two stars r\ [65] and 0. The former 
has a blue companion of the ninth magnitude, 28" distant. 5, one 
of the stars of the rhomboid, is double in an opera-glass, and is sit- 
uated in a fine field. 

Beautiful fields lie between e and R, which is 5° northeast of it. 
The only annular nebula [132] which small telescopes reveal lies one 
third of the way from /3 to 7. It has been described in J 391. 

445. Monoceros, the Unicorn. [Maps III. and II.] — This constel- 
lation contains only four stars as bright as the fourth magnitude. 
It lies east of Orion, and stretches itself in the Milky Way between 
Canis Major and Canis Minor. 

8 [22] lies in the northwestern part of the constellation, at a right 
ascension of 6 h. 19 m. A line from X Ononis (in his head) to a 
Orionis, prolonged 1 g- times its own length, stops just south of 8. 
It is a golden yellow star with a lilac companion of the seventh 
magnitude, 13" distant: it is in a splendid field. 



THE CONSTELLATIONS IN D?:TAIL. 287 

11 [23], which lies in the southwestern part of the constellation, 
has a double companion of the sixth magnitude, 7" distant. The 
components of the companion are 2". 3 apart. It is a star of the 
fourth magnitude, about three eighths of the way from Sirius to 
a Orionis (Betelgueuse), a little east of a direct line. 

2° east of 8, and i° south of the middle point of the line joining 
7 Orionis (Bellatrix) with a Canis Minoris (Procyon) is a cluster 
[99] visible to the naked eye, and very pleasing with a low power. 
Some of the faintest stars are arranged in straight lines. 

A line from Sirius to Canis Majoris, when prolonged three 
fourths of its length, reaches a brilliant coarse cluster [101], in a 
" superb " neighborhood. 

There is a fine field one fifth of the way from 1 1 to 8 ; the fifth 
magnitude star 10 is in it. 

446. Ophiuchus, the Serpent Bearer. [ Map IV. ] — Ophiuchus lies 
between Hercules and Scorpio. The two portions of Serpens lie 
respectively at the east and west sides of this constellation. Ophiu- 
chus is represented as standing on the Scorpion and grasping the 
Serpent with both hands. 

A line from Polaris to fi Draconis (in the Dragon's head), pro- 
longed an equal distance, ends near a, which is in the head of Ophi- 
uchus and near a Herculis. /3 and 7 mark his right shoulder, 1 and k 
the left; v and r are in his right hand, & and e in his left. His right 
knee contains 77 and his left f. The right foot is at 0, the left at p. 

The parallelogram (nearly) formed by f and \ Ophiuchi with a 
and ^ Serpentis is shown by the dotted lines on the map, and is note- 
worthy to the eye : one diagonal of it contains five bright stars. 

\ [51] is a binary, having a period of about 230 years. The com- 
panion is of the sixth magnitude, and is now (1896) i".y distant. 

36 [53], a fifth magnitude star in the southernmost part of the 
constellation, n° east of a Scorpii (Antares), has a sixth magnitude 
companion at a distance of 5". 

7° [58], 4 . 5 east of 7, is a fine binary, completing a revolution 
in less than a century: the seventh magnitude companion is reddish. 
The distance is now (1896) 2" . 

p [49], in the left foot, has an eighth magnitude companion at 
a distance of 4". 

A cluster [120] 3' in diameter lies 9°.5 due east of a Scorpii 






288 DESCRIPTIVE ASTRONOMY. 

(Antares), nearly in line with 36. There are a number of other 
clusters in the vicinity. 

3 south and i° west of f lies a cluster [117] 5' in diameter. 

One third of the way from e to j3 lies a cluster [119] 8' in di- 
ameter, in the centre of which the stars are very closely crowded. 
A line from a to /3 prolonged 2\ times its former length strikes a 
large coarse cluster [128]. 

447. Orion. [Map II.] — Orion is the finest constellation in the 
heavens, and strikes the eye at once : it is best seen in the early 
evening in midwinter. The mighty hunter stands in the attitude of 
smiting Taurus. His belt is formed of three second magnitude stars, 
B, e, and £; it is about 3 in length, and has been called the Ell and 
Yard. Below it dangles the sword, composed of three, or to good 
eyes four, stars in line. The shoulders are marked respectively by 
a (Betelgueuse) and 7 (Bellatrix). In the head is a small isosceles 
right triangle. The left foot is marked by /3 (Rigel), a bluish white 
star of the first magnitude : k occupies the right knee. The right arm 
and club, with which he is to smite Taurus full in the face, are indi- 
cated by the dotted lines going upward from a. The left arm with 
which he holds up the skin of the Nemsean lion, is similarly out- 
lined by a dotted line. 

/3 [15] has a ninth magnitude companion at a distance of io". It 
is not hard to see with a four-inch glass, under good atmospheric 
conditions, and is itself a very close double. 

f [20] is a triple, having a sixth magnitude companion 2". 6 dis- 
tant, and one of the ninth magnitude 57" away. 

1 [17], the southernmost star in the sword, has an eighth magni- 
tude companion at a distance of 12", and one of the tenth magnitude 
49" distant. 

\ [16], in the head, has a companion of the sixth magnitude, 
4" distant. 

<t [18] is a triple star, having a seventh magnitude companion at a 
distance of 42", and one of the eighth magnitude 12" distant: near 
by is a small triangle of three eighth magnitude stars. 

i° south of v, in the right hand, is a cluster [98] of 30 stars of the 
ninth magnitude or fainter. 

A brilliant field [95] lies i° north of 0, containing quite a number 
of stars of the sixth and seventh magnitudes. 



THE CONSTELLATIONS IN DETAIL. 289 

In the sword is the multiple star 6> surrounded by the Great 
Nebula [94], the finest object of its kind in the sky. See § 390. 
In a four-inch telescope the central portion, around the Trapezium, 
can be well seen, in the absence of the moon. 

448. Pegasus, the Winged Horse. [Maps V. and I.]— The chief 
configuration of this constellation is a large rude square which is 
in the body of the horse. A hook-shaped figure starting from one 
corner of the square makes the neck and head of the animal. One 
corner of the square is found by drawing a line from Polaris to 
(3 Cassiopeiae, and prolonging it an equal distance. The star thus 
found is really a Andromedae, but has at times been called 8 Pegasi. 
The neck starts from the opposite corner of the square, and em- 
braces the stars f and £ ; the head starts at 6, and e is in the nose. 

k [79], which is 16 due north of e, has a companion of the elev- 
enth magnitude 12" distant. The main star is a very close double. 

A line from 6 to e, prolonged two thirds of its length, reaches a 
condensed globular star cluster [137], 3 ; or 4' in diameter. 

Midway between € and 6 is a bright group [140]. 

449. Perseus. [Map I.] — Perseus lies between Auriga and An- 
dromeda, a, its chief star, lies on a line from ft Andromedae to 7 
Andromedae, prolonged 1^ times its own length. The most striking 
configuration is the trapezoid of which a is one vertex, from which 
springs a curved line of stars shown by the dotted line on the map. 
9° south of 1 (which is in one corner of the trapezoid) lies /3 (Algol), 
the wonderful variable described in § 379. Near Algol are a few 
stars which form the head of Medusa, the Gorgon which Perseus slew. 
io° southeast of Algol lie a few scattered stars which complete the 
constellation : there is no resemblance to the figure of a man. 

e [12] 'has a lilac companion of the eighth magnitude, at a dis- 
tance of 8". • 

f [10] has three companions of the ninth, tenth, and tenth magni- 
tudes, respectively, at distances of 13", 90", and 122". 

rj [9] has an eighth magnitude companion at a distance of 28". 

Just south of the middle point of a line from 8 Cassiopeiae to 7 
Persei is a large hazy spot, visible to the naked eye even in strong 
moonlight. It is a double cluster [85], the finest object of its class 
in the northern hemisphere. The lowest power should be used in 
viewing it. 

19 



29O DESCRIPTIVE ASTRONOMY. 

i° north of a point five eighths of the way from 7 Andromedae 
to ft Persei (Algol) is one of the finest of low-power fields. 

450. Pisces, the Fishes. [Maps V., II., and I.] — One of the 
Fishes, which is marked by a six-sided polygon, is located just south 
of the square of Pegasus. The star t in this figure forms nearly an 
isosceles right triangle with the two stars a and 7, which form the 
southern side of the square. Thence a ribbon, represented on the 
map by a row of stars connected by a dotted line, extends eastward 
to a, just east of the head of Cetus, thence northward to the other 
Fish, which is an insignificant and chiefly imaginary creature, the 
mouth of which is near ft Andromedae. Though none of the stars are 
especially bright, they are in a dull region, and so are easily traced. 

A line drawn from Polaris through ft Cassiopeiae to a Androm- 
edae (in one corner of the square of Pegasus), and prolonged nearly 
one half of its former length, terminates close by the vernal equi- 
nox, east of the hexagon which marks the southern one of the two 
Fishes. 

a [5] has a companion of the fourth magnitude, distant 3". 

f [2], 12° east and 5° north of a, has an eighth magnitude com- 
panion 23" away. 

451. Piscis Australis, the Southern Fish. [Map V.] — Prolong the 
line of the western edge of the square of Pegasus southward, until 
the prolongation is four times the length of the original line, and a 
(Fomalhaut) will be reached : it is of the first magnitude. The other 
stars of the constellation are then found readily. The constellation 
is too far south for good telescopic views. 

452. Sagitta, the Arrow. [Map V.] — This constellation is just 
north of Aquila and south of Vulpecula. It is a fair representation 
of an arrow, the butt of which is marked by the pretty pair a and ft, 
which lie midway between ft Cygni and a Aquilae (Altair). The 
point of the arrow is at 77. 

f [70] has a companion of the ninth magnitude 9" distant. The 
large star is a very close double. 

6 [71] has two companions, one of the ninth magnitude at a dis- 
tance of n", and one of the eighth, 70" distant. The colors of the 
three stars are called pale topaz, gray, and pearly yellow. 

About a degree south of ft lies a double [6j~\ composed of a ruby 
star of the ninth magnitude, and a blue star of the tenth magnitude, 
20" distant. 



THE CONSTELLATIONS IN DETAIL. 2QI 

Midway between 7 and 8 is a faint but very condensed cluster [133]. 
rj lies in a beautiful low-power field [135], in which are a number 
of doubles. 

453. Sagittarius, the Archer. [Maps V. and IV.] — The conspicu- 
ous part of this constellation looks like a bent bow, with the point of 
the arrow just west of its centre, and the butt 2\ times as far east, in 
one corner of a bright quadrilateral. Sagittarius is a Centaur ; the 
two southern stars of the quadrilateral are in his body. The naked- 
eye double, /3, far to the south, not on the map, marks one of his 
front hoofs. 

A line from Polaris through Vega, prolonged \\ times its former 
length, strikes the quadrilateral. The winter solstice lies 2J south 
and 2° west of (jl, and is i° north of the naked-eye cluster [124]. 

fi [59], in the northwest part of the constellation, has two com- 
panions of the ninth and tenth magnitudes, at respective distances of 
40" and 45". 

Midway between (jl and <j is a cluster [130], 8' in diameter, sur- 
rounded by five stars irregularly placed. It shows well with a four- 
inch glass. 

A line from <x to \ prolonged three fourths of its length termi- 
nates just south of a splendid portion [124] of the Milky Way, 
which well repays examination by its richness. 

3 north of p and i° east is an offshoot [126] of the Milky Way, 
which shows a fine field with a low power. 2° north of p and 5 
east is a brilliant region [129] visible to the naked eye. 

4 north and 1J east of /jl is a very rich field [127]. 

A line from a to fi prolonged three eighths of its length termi- 
nates at a good low-power field [122] containing about 100 stars 
from the ninth magnitude down. 

The line from crtoX prolonged an equal distance stops just south 
of a pair of fifth magnitude stars : close by the northern one is the 
Trifid Nebula [123] described in § 391. A large telescope is re- 
quired to see it well. 

454. Scorpio, the Scorpion. [Map IV.] — A line from Polaris to 
/3 Herculis, prolonged two thirds of its former length, strikes a (An- 
tares), a star of the first magnitude. The downward curve from a 
is easily followed by the eye. At the west of Antares the stars fS, 
8, and ir form a fine curve, like the blade of a scythe, one of the 
handles of which is at a. 



292 DESCRIPTIVE ASTRONOMY. 

a [50] is an elegant double, having a seventh magnitude com- 
panion less than 4" distant. 

ft [46] has a fifth magnitude companion, 13" distant. 

v [47], near ft, has a seventh magnitude companion 40" distant: 
each is a close double. 

£ [45] has a companion of the seventh magnitude, j" away. The 
large star is also double, and may be seen elongated with a four-inch 
telescope without difficulty. 

a [48] has a plum-colored companion of the ninth magnitude, 
20" distant. 

The most condensed mass of stars [116] in the heavens is situ- 
ated half way between a and ft: it lies in a beautiful field, and looks 
like a comet through a small telescope. 

455. Sculptor, the Sculptor. [Maps II. and V.] — This constellation 
lies south of ft Ceti and east of a Piscis Australis (Fomalhaut). It 
is an insignificant group. 

456. Scutum, the Shield. [Map V.] — Scutum is sometimes called 
Clypeus Sobieskii, the Shield of Sobieski ; it is small and inconspic- 
uous, but lies in the thick of the Milky Way : a line from Polaris to 
a Lyrae (Vega), when prolonged nine tenths of its former length, 
ends in Scutum, near the brightest star. There are many faint 
doubles and rich fields. 

457. Serpens, the Serpent. [Map IV.] — The head of the Serpent 
is a triangular figure just south of Corona, between Hercules and 
Bootes. Thence the Serpent's body extends southward through the 
conspicuous parallelogram described in § 446, across Ophiuchus, east 
and northeast, following the dotted line on the map, till it terminates 
at 0, nearly three fourths of the way from ft Ophiuchi to 8 Aquilae. 

8 [43], near the head, has a companion of the fifth magnitude, 
3".6 distant. 

6 [64] has a companion of nearly the same magnitude, 22" 
distant. 

Close by the star 5, which forms a nearly equilateral triangle with 
e and //. in the quadrilateral, is a rich and condensed cluster [115]. 

458. Sextans, the Sextant. [Map III.] — Sextans is an insignifi- 
cant group lying south of the Sickle. A line from 77 Leonis to 
Regulus, prolonged 2\ times its former length, nearly strikes 15, the 
brightest star in the constellation. 



THE CONSTELLATIONS IN DETAIL. 293 

Half a degree north of the middle point of a line joining 8 and 22, 
in the southwest corner of the constellation, is a narrow nebula [107] 
5' long, having a bright nucleus. 

459. Taurus, the Bull. [Map II.]— The face of the Bull is 
marked by a V-shaped figure containing the red first magnitude 
star a (Aldebaran), which is nearly pointed at by the belt of Orion. 
Sirius is as far from the belt on one side as Aldebaran is on the 
other. The horns of the animal are very long, their tips being at 
/3 and f. The well known cluster of the Pleiades is in his fore 
shoulder. Though the latter half of his body is missing, he makes a 
brave feint of charging upon Orion. The V is known as the Hyades : 
one of its stars, 6, is a naked-eye double. 

a [13] has a tenth magnitude companion at a distance of 1 13". 
The Crab Nebula [92] lies i° northwest of £. Through a small 
telescope it is a simple oval. 

460. Triangulum, the Triangle. [Map I.] — The three bright stars 
of this constellation form a right triangle, immediately north of the 
triangle in the head of Aries. 

6, or 1 [7], nearly south of /3, at a distance of 5 , is a " topaz- 
yellow " star of the fifth magnitude, and has a bluish companion of 
the seventh magnitude, 3". 5 distant. 

461. Ursa Major, the Great Bear. [Map I.] —After the Great Dip- 
per has been learned, the rest of the constellation can be made out 
by the help of the dotted lines on the map. The stars h, v, and 
form the head : 1 and k mark one of the fore feet : X and /jl are in 
one of the hind feet, v and f in the other. The stars in the Great 
Dipper have the following names from a to 77 : Dubhe, Merak, 
Phecda, Megrez, Alioth, Mizar, and Benetnasch. The small star 
near Mizar is called Alcor. 

f [38] (Mizar) has a companion of the fifth magnitude, 14" 
distant. 

£ [31] is a rather close and rapid binary, having a period of only 
61 years; the companion is of the fifth magnitude. 

10° north off and 1J nearly east of the fifth magnitude star d is 
a double nebula [105, 106], one component of which is fairly bright: 
they are half a degree apart. 

A line from a to 7, prolonged three fourths of its own length, 
strikes a large oval nebula [no]. 



294 DESCRIPTIVE ASTRONOMY. 

462. Ursa Minor, the Little Bear. [Map I.] — Polaris is the bright- 
est star, and is in the end of the tail. The stars ft, 7, f, and rj are in 
the Bear's body, and form the bowl of the Little Dipper. The 
length of the tail may be ascribed to adaptation to environment. 

a (Polaris) [3] has a companion of the ninth magnitude, 19/' 
distant. 

463. Virgo, the Virgin. [Maps IV. and III.] — The head of the 
Virgin is 5 south of /3 Leonis (Denebola). Thence the body 
stretches east and south to Libra. The lines on the map show its 
general contour. The right arm is graciously extended to take 
in e, and the left hand is given to a (Spica), a star of the first 
magnitude. 

The autumnal equinox lies i° south of the middle point of a line 
connecting (3 and ??. 

7 [35] is a fine binary having a period of 185 years: the com- 
ponents are equal in magnitude, and are now (1896) 5" apart. 

6° north and 4 west of Spica is the triple star 6 [37] ; its com- 
panions are of the ninth and tenth magnitudes, at distances of y" 
and 65" respectively. 

In the wonderful nebulous region of Virgo, bounded by the stars 
/3, 7], 7, S, e, and /3 Leonis, the sky is crowded with nebulae, most of 
which are too faint for small telescopes. One of the brighter ones 
[in] is west of e and 8, forming with them an equilateral triangle. 

464. Vulpecula, the Fox. [Map I.] — Vulpecula contains one star 
of the fourth magnitude, which is 3 J° south of /3 Cygni in the foot of 
the Cross. The rest of the stars are fainter, and most of them lie 
east of the fourth magnitude star, being bounded by Delphinus and 
Sagitta on the south, Cygnus on the north, and Pegasus on the east. 

3J due north from 7 Sagittae, nearly in line with 6 Vulpeculae 
(the brightest star) and 7 Delphini, the Dumb-bell Nebula [134] is 
located : a description of it has been given in § 391. 

USE OF A STAR FINDER OR OF AN EQUATORIAL. 

465. Graduation of the Circles. — In §§ 8-12, directions have been 
given for finding many objects of telescopic interest by the aid of 
the maps. It is often more convenient to find them by means 
of a star finder (Fig. 194), or of a telescope equatorially mounted 



THE CONSTELLATIONS IN DETAIL. 



295 



and provided with an hour circle and a declination circle (Fig. 195). 
Such circles can be affixed to almost any telescope mounting which 




Fig. 194. — The Star Finder. 

is destitute of them by a bright boy of a mechanical turn of mind. 
Two opposite points of the hour circle (the lower one in Fig. 194) 
may be marked o h. and 12 h. 
respectively. Each half of the cir- 
cle would then read o, 1, 2, 3, . . . 
12 h. The circle may then be sub- 
divided into five minute spaces. It 
is well to mark two opposite points 
of the declination circle (the upper 
one in Fig. 194) o°, and to run the 
graduations each side of o° up to 
90 . Each whole degree should 
be indicated. The cut of the star 
finder 1 shows that it is like an 
English equatorial (§ 44), a stick 
taking the place of the telescope. 
When the stick or telescope lies in the plane of the meridian, 
and is perpendicular to the polar axis, the pointer on each circle 

1 A detailed description of this instrument, together with Prof. Wm. A. Rogers's 
method of tracing the constellations by its aid, is given in the " Sidereal Messenger " 
(published by W. W. Payne, Northfield, Minn.) for April, 1889. 




Fig. 195. — The Declination Circle. 



296 



DESCRIPTIVE ASTRONOMY. 



should be opposite the zero of the circle. Both circles of the star 
finder are fast to the polar axis. Any object in the lists at the end 
of this chapter may be found by the star finder, or an equatorial 
telescope, if the sidereal time is known, as will be explained in the 
following sections. 

466. The Sidereal Time at any Instant. — The Nautical Almanac 
gives data and rules for finding with precision the sidereal time at 
any instant, when the mean time is known. The time may be ob- 
tained with sufficient accuracy for present purposes by means of the 
following table and its accompanying explanations. 

Sidereal Time at Mean Noon. 



Jan. 1 


i8h. 


44 m. 


July 


1, 


6h. 


38 m 


" i6 3 


19 h. 


43 m. 


" 


16, 


7 h. 


37m 


Feb. 1 


20 h. 


47 m. 


Aug. 


1, 


8h. 


40 m 


" 16 


21 h. 


46 m. 


" 


16, 


9 h. 


39 m 


March 1 


22 h. 


37 m. 


Sept. 


1, 


10 h. 


42 m 


" 16 


23 h. 


36 m. 


n 


16, 


11 h. 


42 m 


April 1 


oh. 


39 m. 


Oct. 


* 9 


12 h. 


41m 


" 16 


ih. 


38 m. 


a 


16, 


13 h. 


40 m 


May 1 


2h. 


37 m. 


Nov. 


1, 


14 h. 


43 m 


" 16 


3h. 


37 m. 


a 


16, 


15 h. 


42 m 


June 1 


4h. 


40 m. 


Dec. 


1, 


16 h. 


41m 


" 16 


5h. 


39 m. 


tt 


16, 


17 h. 


40 m 



For any date not given in the table, subtract the last preceding 
tabular date from the given date, multiply the difference by 4 m., 
and add the product to the time given opposite the tabular date 
used. 

If the sidereal time at mean noon is required for March 27, the 
last preceding tabular date is March 16; the difference between the 
dates is II days: 11 X4m. = 44m., which added to 23 h. 36m. (the 
time given opposite March 16) gives 24 h. 20 m. As 24 h. is identi- 
cal with o h., we call the answer o h. 20 m. This then is the reading 
of a sidereal clock at noon on March 27. 

To find the sidereal time at 9I1. 23 m. P. M., we reason that, if the 
sidereal time at noon was oh. 20m., and o,h. 23 m. have elapsed 



THE CONSTELLATIONS IN DETAIL. 2gj 

since then, the sidereal time will be found by adding 9 h. 23 m. to 
oh. 20 m., giving 9 h. 43 m. for the time sought. 1 

To find the sidereal time at 7 h. 42 m. P. M., on December 21, we 
reason as follows. At noon of December 16 it was 17 h. 40 m.; 
December 21 is five days thereafter: 5 X4111. = 20m., which added 
to i/h. 40 m. gives 18 h. om. as the sidereal time at noon of 
December 21. Since 7 h. 42 m. have elapsed since noon, we add 
7 h. 42 m. to 18 h. om., obtaining 25 h. 42 m., which is equivalent 
to 1 h. 42 m. 

To find the sidereal time at 3 h. 10 m. A. M., October 24, we 
first notice that the last preceding noon was October 23. The 
sidereal time at noon of October 23 was 13 h. 40m. + 7 x4m., 
which equals 14 h. 8 m. The interval of time between noon of 
October 23 and 3 h. 10 m. A. M. of October 24 was 15 h. 10 m., 
which added to 14 h. 8 m. gives 29 h. 18 m. Since this sum 
is over 24 h. we subtract that from it, and get 5 h. 18 m. for the 
time sought. 

467. The Hour Angle of a Star at any Instant. We can find this 
if we know the right ascension of the star, and the sidereal time at 
the instant at which the hour angle is desired. Suppose that it is 
required to find the hour angle of a Geminorum at 8 h. 15 m. P. M., 
on February 12. From the table in § 466 we find that the sidereal 
time at noon on February 12 was 21 h. 31 m. Then at 8 h. 15 m. 
it would be 5 h. 46 m., as explained in the preceding section. The 
right ascension of a Geminorum is 7 h. 28 m., as given in the list 
of double stars at the end of this chapter. From the discussion in 
§ 131, we see that the hour angle of a star at any instant is the dif- 
ference between its right ascension and the sidereal time at that 
instant. The difference between 5 h. 46 m. and 7 h. 28 m. is 1 h. 
42 m., which is the east hour angle of the star. If the sidereal 
time had been 10 h. 41 m., the hour angle of the star would have 
been 3 h. 13 m., and the star would have been west of the meridian, 
as explained in § 131. 

Astronomers use the following rule for computing the hour angle 
of a star at any instant. 

1 This reasoning is not strictly correct, because sidereal hours are not quite of the 
same length as mean hours. As a sidereal clock goes faster than a mean time clock, it 
will tick off more than gh. while a mean time clock is measuring gh. 



298 DESCRIPTIVE ASTRONOMY. 

Subtract the stars right ascension from the sidereal time at the 
instant. If the remainder is positive, the star is west of the merid- 
ian : if negative, the star is east of the meridian. 

Thus, if the star's right ascension is 1 1 h. 41 m., and the sidereal 
time 8 h. 50 m., the subtraction gives — 2 h. 5 1 m., and the star has an 
east hour angle of 2 h. 51 m. Had the sidereal time been 13 h. 5 m., 
the subtraction would have given -\-i h. 21 m., which would have 
been the west hour angle of the star. 

468. Practical Directions. — In order to find an object with an equa- 
torial, or star finder, it will be advantageous to give heed to the 
following detailed directions, which are based upon the articles im- 
mediately preceding: — 

I. Look up the right ascension and declination of the object 
sought. 

II. Turn the telescope about the declination axis until the read- 
ing of the declination circle equals the declination of the object. 

III. Compute the sidereal time : also the hour angle of the 
object. 

IV. Turn the instrument about the polar axis, not disturbing the 
reading of the declination circle, until the reading of the hour circle 
corresponds to the hour angle just computed. 

V. An eyepiece of low power should be on the telescope. If 
the object is not in the field of view, move the instrument to and fro 
a little around the polar axis. 

469. Lists of Telescopic Objects. — The following telescopic objects 
have been selected because they can be seen with small telescopes. 
Everything in the list will yield to a four-inch telescope : a three-inch 
will show most of them. The right ascensions and declinations are 
given for the year 1900. 



THE CONSTELLATIONS IN DETAIL. 



299 



DOUBLE STARS. 



No. 

I 


Star. 


Right 
Ascen- 
sion. 


Declina- 
tion. 


Mags. 


Dist. 


Notes. 


7} Cassiopeia? 


h. m. 

43 


/ 

+57 17 


4, 


8 


5-5 


Binary : period 200 years. 


2 


£ Piscium 


1 8 


+ 7 3 


5' 


8 


23 




3 


a Ursae Min. 


1 23 


+88 46 


2, 


9 


19 




4 


7 Arietis 


1 48 


-f-18 48 


4^ 


4 


9 




5 


a Piscium 


1 57 


+ 217 


3' 


4 


3 


Fine, on a good night. 


6 


7 Andromedae 


1 58 


+41 5 1 


3' 


6 


11 


Orange, sea-green. 


7 


6 Trianguli 


2 7 


+29 50 


5> 


7 


3-5 


Topaz-yellow, blue. 


8 


7 Ceti 


2 38 


+ 2 49 


3> 


7 


2-5 




9 


77 Persei 


2 43 


+55 29 


4> 


8 


28 




10 


^ Persei 


3 48 


+31 35 


3> 


9 


13 


Two other companions. 


11 


32 Eridani 


3 49 


- 3 i5 


5' 


7 


7 


Topaz-yellow, sea-green. 


12 


e Persei 


3 5i 


+39 43 


3> 


8 


8 


Companion lilac. 


13 


a Tauri 


4 30 


+16 19 


1, 


10 


"3 




14 


14 Aurigae 


5 9 


+32 34 


5' 

5, 


7 

1 r 


14 
J 3 


Triple. 


15 


£ Orionis 


5 10 


- 8 19 


1, 


9 


10 




16 


A Orionis 


5 30 


+ 9 52 


3' 


6 


4 




17 


1 Orionis 


5 3i 


- 5 59 


3> 


8 


12 


A faint companion 49''' away. 


18 


o- Orionis 


5 34 


- 2 39 


4, 
4. 


7 

8 


42 
12 


Fine triple. 


19 


45 Leporis 


5 35 


-17 54 


7 




Multiple star. 


20 


£ Orionis 


5 36 


— 2 


2, 


6 


2.6 


A faint companion 57" away. 


21 


7 Leporis 


5 4o 


—22 29 


4> 
4, 


7 

r 1 


93 

45 




22 


8 Monocerotis 


6 18 


+ 4 39 


5' 


7 


r 3 


Golden-yellow and lilac. 


23 


1 1 Monocerotis 


6 24 


- 6 58 


5- 


6 


7 


Companion double. 


24 


ix Canis Maj. 


6 52 


-13 55 


5> 


9 


3-5 




2 5 


8 Geminorum 


7 14 


+22 10 


3' 


8 


7 




26 


Geminorum 


7 28 


+32 6 


2, 


3 


5-5 


Magnificent double. 


27 


£Cancri 


8 6 


+17 57 


5, 


7 


5 


Main star close double. 


28 


e Hydrae 


8 41 


+ 6 47 


3> 


8 


3-5 


Companion blue. 


29 


38 Lyncis 


9 13 


+37 14 


4, 


7 


2.8 


Companion lilac. 


30 


7 Leonis 


10 14 


+20 21 


2, 


4 


3-5 


Binary : period 400 years. 


3 1 


£ Ursae Maj. 


11 13 


+32 6 


4, 


5 


2 


Binary : period 61 years. 


32 


t Leonis 


11 19 


+" 5 


4> 


7 


2.8 




33 


2 Can. Ven. 


12 11 


+4i 13 


5, 


9 


11 


Orange and blue. 


34 


8 Corvi 


12 25 


-15 58 


3> 


8 


24 




35 


7 Virginis 


12 37 


- 54 


3> 


3 


5-5 


Binary : period 185 years. 


36 


Can. Ven. 


12 51 


+38 51 


3' 


6 


20 




37 


6 Virginis 


13 5 


-50 


4- 


9 


7 


Another companion 65" away. 


38 


C Ursae Maj. 


13 20 


+55 27 


2, 


5 


14 




39 


t Bootis 


14 13 


+5i 50 


4» 


8 


3? 




40 


7r Bootis 


H 3 6 


+16 51 


4> 


6 


6 




41 


e Bootis 


14 41 


+27 30 


3. 


6 


3 


Slow binary : yellow, blue. 


42 


| Bootis 


H 47 


+19 31 


5. 


7 


4 


Binary : companion purple. 


43 


8 Serpentis 


15 3o 


+10 52 


4» 


5 


3-6 




44 


£ Coronas 


l S 36 


+ 3 6 58 


4. 


6 


6 




45 


| Scorpii 


x 5 59 


-11 6 


5> 


7 


7 


Main star double. 


46 


j8 Scorpii 


16 


-19 32 


2, 


5 


13 




47 


v Scorpii 


16 6 


—19 12 


4. 


7 


40 


Each star a close double. 


48 


<r Scorpii 


16 15 


-25 21 


3« 


9 


20 


Companion plum-colored. 


49 


p Ophiuchi 


16 20 


— 23 13 


5' 


8 


4 





[OO 



DESCRIPTIVE ASTRONOMY. 



No. 

5° 


Star. 


Right ! 
Ascen- 
sion. 


Declina- 
tion. 


Mags. 


Dist. 


Notes. 


Scorpii 


h. m. 
16 23 


-26 I 3 


I, 7 


3-7 


Fiery red, green. 


51 


A. Ophiuchi 


16 26 


+ 2 12 


4, 6 


i-7 


Binary : period 230 years. 


5 2 


yu. Draconis 


T 7 3 


+54 36 


5, 5 


2.6 




53 


36 Ophiuchi 


17 9 


-2627 


5, 6 




54 


Herculis 


17 10 


+14 3° 


2, 6 


5 


Companion emerald. 


55 


5 Herculis 


17 11 


+24 57 


3, 8 


19 




56 


39 Ophiuchi 


17 12 


—24 11 


6, 8 


12 


Orange, blue. 


57 


p Herculis 


17 20 


+37 14 


4, 5 


4 


Binary. 


58 


70 Ophiuchi 


18 


+ 2 32 


4, 7 


2 


Binary. 


59 


fi Sagittarii 


18 8 


-21 5 


4, 9 


40 


Another companion 45" away. 


60 


a Lyrae 


18 34 


+38 41 


1, 10 


48 




61 


e Lyrae 


18 41 


+39 34 


4. 5 

5, 5 


3-3 

2-5 


Quadruple. 


62 


C Lyras 


18 41 


+37 30 


4, 5 


40 




63 


)8 Lyrae 


18 46 


+33 l S 


4, 8 


46 


Two other companions : main star 


64 


9 Serpentis 


18 51 


+ 44 


4, 4 


22 


[variable. 


65 


77 Lyrae 


19 10 


+38 58 


4, 9 


28 


Companion blue. 


66 


£ Cygni 


19 27 


+27 45 


3» 7 


34 


Fine colored double. 


67 


Anon. Sagittas 


19 36 


+16 20 


9, 10 


20 


Ruby and blue. 


68 


17 Cygni 


l 9 43 


+33 30 


6, 9 


26 




69 


7r Aquilae 


19 44 


+11 34 


6, 6 


i-5 




70 


£ Sagittae 


l 9 45 


+18 53 


5. 9 


9 


Main star a very close double. 


71 


9 Sagittas 


20 6 


+20 37 


6, 9 


11 


Another companion 70" away. 


72 


7r Capricorni 


20 22 


-18 32 


5. 9 


3 


Difficult with four-inch glass. 


73 


p Capricorni 


20 23 


-18 9 


5, 9 


3-8 




74 


7 Delphini 


20 42 


+15 46 


3, 6 


11 


Yellow, greenish blue. 


75 


e Equulei 


20 54 


+ 3 55 


5^ 7 


11 


Rapid binary. 


76 


61 Cygni 


21 2 


+38 13 


5, 6 


20 


The first star the distance of which 
from us was found. 


77 


)3 Cephei 


21 27 


+70 7 


3, 8 


14 


Companion blue. 


78 


M Cygni 


21 40 


+28 18 


4, 5 


4 




79 


k Pegasi 


21 40 


+25 11 


4> 11 


12 


Main star a very close double. 


80 


£ Cephei 


22 1 


+64 8 


5» 7 


6 


Companion blue. 


81 


£ Aquarii 


22 24 


- 32 


3. 3 


3 




82 


8 Cephei 


22 25 


+57 54 


4, 7 


4 

1 


Yellow and blue : a noted variable. 

1 



CLUSTERS AND NEBULAE. 



No. 


Constellation. 


Right 
Ascen- 
sion. 


Declina- 
tion. 


Description. 


83 
84 

85 
86 

87 
88 

89 
90 


Andromeda 

Cassiopeia 

Perseus 

Perseus 

Taurus 

Taurus 

Camelopardus 

Auriga 


h. m. 

37 

1 27 

2 12 

2 3 6 

3 42 

4 14 

4 54 

5 1 


/ 

+40 43 
+60 10 
+56 41 
+42 18 

+23 48 
+ l 5 23 
+53 43 
+37 14 


The Great Nebula : visible to the naked eye. 

Beautiful field of stars. 

Magnificent double cluster. 

Fine field. 

The Pleiades. 

The Hyades. 

Rich coarse cluster. 

Rich field. 



THE CONSTELLATIONS IN DETAIL. 



30 



No. 


Constellation. 


Right 
Ascen- 
sion. 


Declina- 
tion. 


Description. 


91 


Auriga 


h. m. 

5 23 


/ 

+35 45 


Fine cluster. 


92 


Taurus 


5 28 


+21 57 


The Crab Nebula. 


93 


Auriga 


5 30 


+34 5 


B right cluster. 


94 


Orion 


5 3° 


- 5 27 


Great Nebula in Orion. 


95 


Orion 


5 3 1 


- 4 25 


Brilliant field. 


96 


Auriga 


5 46 


+32 3 1 


Large cluster of faint stars. 


97 


Gemini 


6 3 


+24 26 


Cluster, visible to the naked eye. 


98 


Orion 


6 4 


+13 58 


Cluster of 30 stars. 


99 


Monoceros 


6 26 


+ 4 56 


Coarse cluster, visible to the naked eye. 


100 


Canis Maj. 


6 43 


-20 38 


Superb naked-eye cluster. 


IOI 


Monoceros 


6 58 


- 8 12 


Bright coarse cluster. 


102 


Argo 


7 32 


—14 16 


Diffuse group, visible to the naked eye. 


103 


Argo 


7 37 


-14 29 


Circular cloud of small stars. 


104 


Cancer 


8 34 


+20 17 


Praesepe. 


r °5 


Ursa Maj. 


9 47 


+69 36 


Elliptical nebula. 


106 


Ursa Maj. 


9 47 


+70 18 


Long narrow nebula. 


107 


Sextans 


10 


- 7 14 


Narrow nebula 5' long. 


108 


Hydra 


10 20 


-18 8 


Planetary nebula. 


109 


Leo 


10 43 


+ r 3 J 3 


Double nebula. 


no 


Ursa Maj. 


12 14 


+47 5 1 


Large oval nebula. 


in 


Virgo 


12 25 


+ 8 33 


Nebula, fairly bright. 


112 


Coma Ber. 


13 8 


+18 42 


Condensed cluster of faint stars. 


"3 


Can. Ven. 


13 26 


+47 42 


Great Spiral Nebula. 


114 


Can. Ven. 


13 38 


+28 52 


Cluster containing over 1,000 stars. 


"5 


Serpens 


15 x 3 


+ 2 28 


Rich globular cluster. 


116 


Scorpio 


16 11 


-22 45 


Most condensed cluster known. 


117 


Ophiuchus 


16 27 


—12 50 


Rich cluster. 


118 


Hercules 


16 38 


+36 39 


Superb globular cluster. 


119 


Ophiuchus 


16 42 


- 1 47 


Cluster : stars crowded at centre. 


120 


Ophiuchus 


16 56 


-26 8 


Condensed cluster of rather faint stars. 


121 


Hercules 


17 14 


+43 H 


Very condensed cluster. 


122 


Sagittarius 


17 5i 


-18 59 


Cluster of 100 stars. 


123 


Sagittarius 


17 56 


-23 2 


The Trifid Nebula. 


124 


Sagittarius 


17 58 


-24 23 


Naked eye cluster. 


125 


Draco 


17 59 


+66 38 


Bright planetary nebula. 


126 


Sagittarius 


18 12 


-18 27 


Cluster visible to the naked eye. 


127 


Sagittarius 


18 14 


—17 n 


Very rich field. 


128 


Ophiuchus 


18 23 


+ 6 30 


Large coarse cluster. 


129 


Sagittarius 


18 26 


-19 9 


Brilliant region. 


130 


Sagittarius 


18 30 


-23 59 


Fine, rather faint cluster. 


131 


Aquila 


18 46 


- 6 23 


Fan-shaped cluster of faint stars. 


132 


Lyra 


18 50 


+32 54 


Large annular nebula. 


133 


Sagitta 


19 49 


+18 31 


Faint condensed cluster. 


134 


Vulpecula 


l 9 55 


+22 27 


The Dumb-bell Nebula. 


i35 


Sagitta 


20 1 


+19 42 


Fine field about 77 Sagittae. 


136 


Aquarius 


20 59 


-11 45 


Greenish blue planetary nebula. 


i37 


Pegasus 


21 25 


+11 43 


Condensed globular cluster. 


138 


Aquarius 


21 28 


- 1 16 


Compact faint globular cluster. 


139 


Cygnus 


21 29 


+47 59 


Large cluster in rich vicinity. 


140 


Pegasus 


21 52 


+ 7 10 


Bright group. 


141 


Lacerta 


22 12 


+49 23 


A fair cluster. 


142 


Cassiopeia 


23 52 


+56 10 


Large cloud of minute stars. 



302 



DESCRIPTIVE ASTRONOMY. 



A FEW COLORED STARS AND VARIABLES. 



No. 


Constellation. 


Right 
Ascen- 
sion. 


Declina- 
tion. 


Description. 


J 43 


Cetus 


h. m. 
2 14 


' 
- 3 26 


Mira : see § 376. 


144 


Cetus 


2 57 


+ 3 42 


a Ceti: orange star. 


H5 


Perseus 


3 2 


+40 34 


Algol : see § 379. 


146 


Taurus 


4 45 


+28 21 


Crimson : eighth magnitude. 


H7 


Lepus 


4 55 


-14 57 


R Leporis : crimson variable : magnitude 6-9. 


148 


Lynx 


6 18 


+58 28 


5 Lyncis : fiery red, in fine group : sixth mag. 


149 


Auriga 


6 30 


+38 32 


Deep red : sixth magnitude. 


J 5o 


Argo 


7 29 


-14 18 


Red: sixth magnitude, fine neighborhood. 


I 5 I 


Leo 


9 42 


+11 54 


R Leonis : crimson variable : magnitude 5-10. 


T 5 2 


Crater 


10 56 


-17 47 


R Crateris : red variable : magnitude 8-9. 


153 


Libra 


14 56 


-8 7 


5 Librae : variable : period z\ days. 


i54 


Libra 


15 12 


- 9 1 


£ Librae : pale green. 


J 55 


Corona 


15 55 


+26 12 


T Coronae : a famous variable. 


156 


Lyra 


18 29 


+3 6 55 


Decided crimson : eighth magnitude. 


T 57 


Aquila 


19 47 


T045 


77 Aquilae : variable : magnitude 4-5. 


158 


Cassiopeia 


23 53 


+50 50 


R Cassiopeiae : very red variable : magn. 5-12. 



APPENDIX I. 



303 



APPENDIX I. 

470. NAMES OF STARS. 

The following list contains the proper names of some of the prominent 
stars, together with their corresponding designations in the Greek letter 
nomenclature. 



A-cher'-nar . 
Al-bl'-re-o 
Al-cy'-o-ne . 
Al-deb'-a-ran 
Al'-ge-nib . 
Al'-ge-nib (sometimes) 
Al'-gol . . 
Ar-i-oth . . 
Al'-kaid . . 
Al'-phard . 
Al-phec'-ca . 
Al'-phe-ratz 
Al'tair . . 
Ant-ar'-es (ez) 
Arc-tu'-rus . 
Ar'-i-ded 
Bel'-la-trix . 
Be-net'-nasch 
Betelgueuse (Be'-tel 
Ca-p el-la 
Caph . . . 
Cas'-tor . . 
Cor Car'-o-li 
Cor Hy'-dras 
Cor Le-6'-nis 
Cor Ser-pen'-tis 



a Eridani 

£ Cygni 

77 Tauri 

a Tauri 

7 Pegasi 

a Persei 

)8 Persei 

€ Ursas Majoris 

7? Ursas Majoris 

. . a Hydras 

o Coronas Bor. 

o Andromedas 

. . o Aquilas 

. . a Scorpii 

. . a Bootis 

. . a Cygni 

. . 7 Orionis 

77 Ursas Majoris 

. . a Orionis 

. . a Aurigas 

£ Cassiopeia? 

a Geminorum 

. o Can. Ven. 

. . a Hydras 

. . o Leonis 

. o Serpentis 



De'-neb ......... o Cygni 

De-neb'-o-la # Leonis 

Dub'-he o Ursas Majoris 

E'-nif e Pegasi 

Fomalhaut (Fo'-mal-o) a Piscis Australis 

Gem'-ma ... a Coronas 

Ham'-al o Arietis 

Ko'-chab £ Ursas Minoris 

Mar'-kab a Pegasi 

Me'-grez 8 Ursas Majoris 

Mi'-ra Ceti 

Mi'-rach # Andromedas 

Mi'-zar ( Ursas Majoris 

Phec'-da 7 Ursas Majoris 

Po-la'-ris a Ursas Minoris 

Pol'-lux /8 Geminorum 

Pro'-cy-on a Canis Minoris 

Ras'-al-hag'-ue a Ophiuchi 

Reg'-u-lus a Leonis 

Rigel (Ri'-ghel) Orionis 

Scheat £ Pegasi 

Sir'-i-us a Canis Majoris 

Spf-ca a Virginis 

Thu'-ban a Draconis 

Ve'-ga a Lyras 



r 



304 DESCRIPTIVE ASTRONOMY. 



APPENDIX II. 

471. ASTRONOMICAL CONSTANTS. 

d. h. m. s. 

Sidereal Year 365 6 9 8.97 

Tropical Year 365 5 4-8 45 5 1 

Sidereal Month 27 7 43 n-54 

Synodic Month 29 12 44 2.68 

h. m. s. 

Sidereal Day ... 23 56 4.090 of mean solar time. 
Mean Solar Day . . 24 3 5 6 -5S 6 of sidereal time. 

Obliquity of the Ecliptic 23 27' 8".o 

Constant of Precession 50" '.264 

Constant of Aberration 2o"492 

The lengths of the year, the obliquity of the ecliptic, and the constant of 
precession are given for the year 1900. The lengths of the year and of the 
month are given in mean solar time. 






PLANETARY DATA. 



305 



APPENDIX III. 

472. PLANETARY DATA. 



Planet. 


Mean Distance, 
the Earth's 
being Unity. 


Mean Dis- 
tance, Millions 
of Miles. 


Sidereal 
Period. 


Eccentricity 
of Orbit. 


Inclination of 
the Orbit to 
the Ecliptic. 


Mercury 


O.387099 


36.O 


d. 
87.969 


O.2056 


/ 

7 


Venus 


0723332 


67.2 


224.701 


O.O068 


3 2 4 


The Earth 


I.COOOOO 


929 


365.256 


O.OI68 





Mars 


I. 523691 


I4I-5 


686.980 


O.0933 


1 5 1 


Jupiter 


5.202800 


4S3-3 


11.86 


O.O483 


1 19 


Saturn 


9.538861 


886.1 


29.46 


O.0561 


2 30 


Uranus 


19.18329 


1 782.1 


84.02 


O.0463 


46 


Neptune 


30.05508 


2792.0 


164.78 


O.OO90 


1 47 



Planet. 


Mean 
Diameter 

in Miles. 


Mass, the 

Sun's being 

Unity. 


Density, 

the Earth's 

being 

Unity. 


Time of 
Rotation. 


Inclination 
of Equator 
to Orbit. 


Superficial 
Gravity, the 
Earth's be- 
ing Unity. 


Mercury 
Venus 
The Earth 
. Mars 
Jupiter 
Saturn 
Uranus 
Neptune 


3>°3° 

7,700 

7,918 

4,230 

88,000 

73,000 

31,900 

34,800 


1 


2.21 
O.86 
1. 00 
O.72 
O.24 
O.I3 
0.22 
0.20 


88 days 

225 days 
h. m. s. 

23 56 4.09 

24 3 7 22.67 

9 55 
10 14 24 
Unknown 
Unknown 


0° (?) 
o° (?) 

23° 27' 

24° 5o' 
3° 5' 

26 49' 
Unknown 
Unknown 


O.S5 
O.S3 
I. OO 
O.3S 

2.65 

1.1S 
0.91 
o.SS 


2,668,700 

1 


425,000 
1 


331,100 

1 


3,104,700 

1 
1048 

1 

3486 

1 


22765 
1 


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LANDMARKS IN THE HISTORY OF ASTRONOMY. 307 



APPENDIX IV. 

473. LANDMARKS IN THE HISTORY OF ASTRONOMY. 

The data to be given under this heading may serve to outline, though in a 
rude and imperfect way, the historical development of the science of astron- 
omy. Many suggestions for essays to be written by the students may be 
derived from it. 

Herodotus declared (with his customary accuracy !) that the Egyptians 
had made astronomical observations for more than 11,000 years, and had seen 
the ecliptic perpendicular to the equator. Though these statements are man- 
ifestly untrue, they indicate that the Egyptians cultivated astronomy from a 
remote antiquity. Diodorus states that they were able to calculate eclipses. 
Their year consisted of 365 days : religious ceremonies were regulated by 
the phases of the moon. As their writings are lost, the real extent of their 
knowledge is largely a matter of conjecture. The gigantic Pyramid of Che- 
ops, set square with the points of the compass, silently testifies to astronomical 
knowledge on the part of its unknown builders. 

The Chaldeans lived in and about Babylon. Porphyry states that Callis- 
thenes transmitted to Aristotle a series of Babylonian observations reaching 
back to 2200 b. c. But Ptolemy, who made use of the Chaldean observations, 
quotes none prior to 720 b. c. They learned to predict eclipses by means of 
their discovery of the eighteen-year cycle, called the Saros : similar series of 
eclipses recur in successive cycles. 

The Hindoos seem to have possessed an extensive knowledge of astronomy 
in olden times. But the dates of their ancient writings are very uncertain. 
Some believe that their knowledge was derived from the Greeks, while others 
assign to it a much higher antiquity. There are references in their writings 
to a conjunction of the planets which took place 5,000 years ago. It is sup- 
posed that their knowledge of it was not obtained by observation, but by cal- 
culating backwards. Modern tables show that the conjunction was far from 
exact. 

The Chinese refer the beginning of their astronomical observations to a 
date about 3000 b. c. Over 5,700 years ago the Emperor Fou-Hi is reputed 
to have been a diligent student of astronomy. About 2600 b. c. Hoang-Ti, 
likewise an emperor, is said to have built an observatory, and to have estab- 
lished a mathematical tribunal for the purposes of correcting the calendar and 



308 DESCRIPTIVE ASTRONOMY. 

predicting eclipses. It is stated that a solar eclipse, which occurred some 4,000 
years ago, was of more than usual interest to Hi and Ho, two imperial astron- 
omers, who failed to predict it, and so lost their heads through heedlessness. 
It seems probable that the records after 720 b. c. are authentic. They em- 
brace accounts of eclipses and of remarkable comets. 

B. C. 640-546. Thales, the chief of the Seven Sages, flourished. He was 
the founder of the Greek school of astronomy ; he taught that the moon 
receives its light from the sun, while the stars are self-luminous ; he believed 
that the earth was a sphere. 

B. C. 611-547. Anaximander, of Miletus, was the immediate successor 
of Thales in the school of Ionian philosophers. He was distinguished for his 
wide knowledge of astronomy and geography, and is thought to have intro- 
duced the sun dial into Greece. 

B. C. 569-470. Pythagoras, of Samos, is reputed to have taught his dis- 
ciples that the earth was not the centre of the universe, but that there was a 
central fire about which the sun, moon, earth, planets, and stars revolved. 

B. C. 520-460. Parmenides, who lived about this time, wrote a poem on 
Nature, fragments of which have come down to us. He is said to have 
taught that the earth was a sphere, and that Lucifer, the morning star, was 
the same body as Hesperus, the evening star. 

B. C. 500-428. Anaxagoras, of Clazomenae, an intimate friend of Peri- 
cles, was another philosopher of the Ionian school, who explained eclipses 
correctly. 

B. C. 469-399. Socrates used his influence against the study of astron- 
omy, except for the practical purposes of surveying and determination of 
time. This was, however, a mere incident in his teaching, which was chiefly 
directed to moral ends. 

B. G. 460. Diogenes, of Apollonia, asserted that the oblique position of 
the earth's axis with reference to the plane of its orbit was a cause of the 
changes of the seasons. 

B. C. 433. Meton, an Athenian, discovered the " Metonic Cycle," which 
is still used in finding the time of Easter. The cycle embraces 235 synodic 
months, which are almost exactly equal to 19 years of 365^ days each. This 
cycle is also of use in predicting eclipses. 

B. C. 366. Eudoxus is said by Pliny to have introduced into Greece the 
common year of 365^ days. 

B. C. 340. Autolycus, of yEolis, wrote two astronomical works, the oldest 
extant specimens of astronomical writing. 

B. C. 330. Pytheas, a noted Greek navigator, pointed out the fact that 
there was a connection between the tides and the moon. 

B. C. 287-212. Archimedes made great strides in pure and applied 



LANDMARKS IN THE HISTORY OF ASTRONOMY. 309 

mathematics : he attempted to measure the sun's diameter. A planetarium 
which he constructed was celebrated in its day. 

B. C. 280. Aristarchus, of Samos, flourished. He was the first to main- 
tain that the earth revolved about the sun : he also devised a correct method 
of determining the relative distances of the sun and moon from the earth. 

B. C. 276-196. Eratosthenes, of Cyrene, determined the obliquity of the 
ecliptic, and was the first to attempt to measure the magnitude of the earth 
by a correct method. 

B. C. 190-120. Hipparchus, of Nicsea in Bithynia, did the memorable 
work which has given him the appellation of the Father of Astronomy. He 
was the first to use right ascensions and declinations, and made the earliest 
catalogue of stars. The solution of plane and spherical triangles by principles 
closely akin to those now employed in trigonometry, was first accomplished 
by him. He devised the method of locating places on the earth by latitude 
and longitude, discovered the precession of the equinoxes, calculated eclipses, 
divided the day into periods of twelve hours each, and determined the pe- 
riods of the planets. 

A. D. 100-170. Ptolemy, the Alexandrian astronomer, produced a num- 
ber of astronomical and geographical works, the most celebrated of which is 
now known as the Almagest, a name given by the Arabians. The theory of 
the celestial motions which he advocated is known as the Ptolemaic theory, 
and enthralled astronomers for 1,400 years. It placed the earth, a motion- 
less sphere, in the centre of the universe, which revolved about it. 

A. D. 415. The modest and beautiful Hypatia, daughter of Theon, an 
Alexandrian philosopher, was murdered. She was the first woman known to 
have been profoundly versed in mathematics. 

A. D. 877-929. Albategnius, an Arabian astronomer, made his observa- 
tions : he was the most accomplished astronomer from the days of Hippar- 
chus to those of Tycho Brahe. He made a star catalogue, and obtained 
more exact values of the annual precession and of the obliquity of the ecliptic 
than had been known previously. 

A. D. 1214-1294. Roger Bacon laid the foundations of the modern exper- 
imental method in science, afterwards elaborated by Francis Bacon : he was 
a conspicuous champion of intellectual liberty. His researches in optics, if 
pursued a little further, might have led him to the invention of the telescope. 

A. D. 1288. The first important public clock in England was erected : 
the pendulum was not yet applied to timepieces. 

A. D. 1436-1476. John Mtiller, of Konigsberg, better known as Regio- 
montanus, brought trigonometry to a high degree of advancement, wrote 
several valuable works, calculated the places of the planets for many years to 
come, and improved the imperfect clocks then in use. 



3IO DESCRIPTIVE ASTRONOMY. 

A. D. 1543. The great work of Copernicus, entitled De Revolutionibus 
Orbium Ce.':stiu?n. was published. This work set forth the theory that the 
sun was the centre of the solar system. The theory gained acceptance only 
after a sturdy battle with the adherents of the Ptolemaic system, which had 
been generally believed for fourteen centuries. 

A. D. 1576. Tycho Brahe. a Dane, begins the construction of the splen- 
did observatory called Uranibuig, erected upon an island in the Baltic Sea 
through the munificence of Frederick, the king of Denmark. The instru- 
ments which he employed were much more accurate than any others that had 
ever been constructed. The study of his observations of the planets led 
Kepler to the discovery of his famous laws. 

A. D. 1583. Galileo, of Pisa, noticed that the vibrations of a pendulum 
were isochronous (performed in equal times). This he discovered by ob- 
serving the oscillations of a great bronze lamp suspended from the ceiling of 
the cathedral in his native town. 

A. D. 1596. Fabricius discovered the variability of Mira. 

A. D. 1600. Giordano Bruno, who had unsparingly exposed many of the 
absurdities of the Aristotelian system of natural philosophy, had pointedly 
ridiculed it. and had propagated heretical notions, (as. for instance, that the 
stars were suns,) was burned at Rome. 

A. D. 1608. Hans Lippershey. of Middleburg in Holland, invented the 
refracting telescope. 

A. D. 1609. Kepler published his " Treatise on the Motion of the Planet 
Mars," in which his first and second laws of planetary motion were enun- 
ciated. Galileo made a telescope having a concave lens for the eyepiece : 
opera-glasses have such eyepieces. 

A. D. 1610. Galileo announced that his telescope had revealed moons 
accompanying Jupiter, mountains on the moon, the phases of Venus, etc. 

A. D. 1613. Galileo published his discovery of spots on the sun : by 
observations :f them he detected its rotation. 

A. D. 1614. Napier, a Scotchman, published his Mirifici Logarith- 
: Canonis Desc iptio. Though he is the inventor of logarithms, those 
commonly employed in calculation are due to his friend Briggs. 

A. D. 1618. Kepler discovered his third law of planetary motion. 

A. D. 1631. The first recorded transit of Mercury was observed by Gas- 
sendi, one of the most eminent of Galileo's disciples. 

A. D. 1638. Gascoigne invented and used the filar micrometer. The 
present precision of observations of the places of the celestial bodies is lar_r/ 
due to the use of this instrument. 

A. D. 1639. A transit of Venus was observed for the first time : Horrocks 
and Crabtree were the observers. 



LANDMARKS IN THE HISTORY OF ASTRONOMY. 311 

A. D. 1655. Huyghens discovered that the mysterious appendage of 
Saturn was a ring. 

A. D. 1656. Huyghens made the first pendulum clock, thus giving to 
astronomers the priceless boon of an accurate instrument for measuring 
time. 

A. D. 1663. James Gregory, a Scotch professor, invented the form of the 
reflecting telescope which bears his name (the Gregorian form) . 

A. D. 1675. Romer, a Dane, the inventor of the transit instrument, an- 
nounced that light occupied time in traversing the celestial spaces, and deter- 
mined its velocity roughly. 

A. D. 1687. Newton published the Principia, universally conceded to be 
the masterpiece of the world's scientific thought. 

A. D. 1705. Halley predicted that the Great Comet of 1682 would re- 
turn in 1759. 

A. D. 1727. Bradley, an English astronomer, discovered the aberration 
of light. 

A. D. 1731. Halley invented the sextant, which has proved invaluable to 
mariners. 

A. D. 1758. Dollond, an English optician, invented the form of achro- 
matic object-glass now generally used. 

A. D. 1765. Harrison, an English watchmaker, finally obtained a portion 
of the reward offered by Parliament for improvement in watches for the 
benefit of navigation. His chronometer ran very well, but was much larger 
than the chronometers of to-day. He was the inventor of the " gridiron " 
pendulum. 

A. D. 1781. Sir William Herschel discovered the planet Uranus. 

A. D. 1795. Rehabilitation of the French Academy of Sciences, as a 
branch of the Institute. The latter part of the eighteenth century and the 
early years of the nineteenth were distinguished by many profound and ele- 
gant mathematical researches concerning the movements of the bodies com- 
posing the solar system ; special attention was paid to the perturbations due 
to their mutual attractions. Foremost among the investigators were Laplace 
and Lagrange, the most eminent of French mathematicians. 

A. D. 1801. Piazzi, of Palermo, discovered Ceres, the first minor planet. 

A. D. 1803. Sir William Herschel published his discovery that certain 
double stars have a motion of revolution about their common centre of 
gravity. 

A. D. 1840. The moon was first photographed by Dr. J. W. Draper, of 
New York. 

A. D. 1846. The planet Neptune was discovered : this is esteemed the 
greatest triumph of mathematical analysis. 



312 DESCRIPTIVE ASTRONOMY. 

A. D. 1859. Spectrum analysis, which has lately yielded marvellous re- 
sults, entered the service of astronomy. 

A. D. 1867. The orbit of the November meteor showers was proved to 
be practically identical with that of TempePs comet. 

A. D. 1868. The sun's prominences were observed by Janssen and 
Lockyer by means of the spectroscope, in full sunshine : they had hitherto 
been seen only during total solar eclipses. 

A. D. 1877. The satellites of Mars were discovered by Professor Asaph 
Hall, with the twenty-six inch telescope of the United States Naval Observa- 
tory, at Washington. 

A. D. 1887. An International Photographic Congress, meeting at Paris, 
decided upon a plan for photographing the entire heavens. 

A. D. 1892. The fifth satellite of Jupiter was discovered by Barnard. 

A. D. 1895. Saturn's rings were spectroscopically proved by Keeler to 
be composed of small bodies. Helium was found to be widely disseminated 
throughout the universe. 



TOPICS FOR ESSAYS. 313 



APPENDIX V. 

474. TOPICS FOR ESSAYS. 

The following subjects are suggested for essays. The topics cover a wide 
range, and are of various degrees of difficulty. 

1. The Astronomy of the Chinese. 

2. The Astronomy of the Chaldeans. 

3. Astronomy among the Ancient Hindoos. 

4. The Ancient Greek Astronomy (especially the work of Thales, Pythagoras, 
and Hipparchus). 

5. The Astronomical Work of Ptolemy (explaining particularly the system of 
cycles and epicycles embraced in the Ptolemaic theory). 

6. The Debt of Astronomy to the Arabians. 

7. The Origin of the Constellations. 

8. Ancient Ideas of the Nature and Movements of the Heavenly Bodies. 

9. Ancient Ideas of the Shape, Support, and Motion of the Earth. 

10. The Reckoning of Time among Ancient Nations. 

11. Astrology. 

12. Astronomical References in the Bible. 

13. Copernicus. 

14. Tycho Brahe. 

15. Kepler. 

16. Galileo. 

17. Newton. 

18. Laplace. 

19. The Herschels (William, Caroline, and John). 

20. Growth of Knowledge of the Planetary Motions (especially the relation 
between the advances made by Copernicus, Tycho, Kepler, and Newton). 

21. Invention and Development of the Refracting Telescope. 

22. Invention and Development of the Reflecting Telescope. 

23. Astronomical Spectroscopy. 

24. Astronomical Photography. 

25. The Nebular Hypothesis. 

26. Habitability of other Worlds. 

27. Does Astronomical Research tend to produce Scepticism ? 

28. The Characteristics of an Ideal Astronomer. 

29. Geodesy, or the Measurement of the Earth's Form and Dimensions. 



314 DESCRIPTIVE ASTRONOMY. 

30. The Work of the United States Coast Survey. 

31. The System of Standard Time in the United States, and its Advantages. 

32. Methods of Measuring the Velocity of Light- 

33. The Decay of the Universe. 

34. The Magnitude of the Forces at work in the Sun. 

35. Pending Problems in Astronomy. 

36. The Making of a Modern Object-glass. 
yj. The Stability of the Solar System. 

38. The Great Telescopes of the World. 

39. The Evolution of the Moon from the Earth. 

40. Electricity as a Handmaid of Astronomy. 

41. Personal Equation. 

42. An Ideal Site for an Observatory. 

43. Usefulness of Astronomy. 

44. The Mental Training to be derived from the Study of Astronomy. 

45. Foucault's Pendulum Experiment. 

46. History of Clocks and Watches. 

47. Progress of Astronomy during the Nineteenth Century. 

48. The Moon and the Weather. 

49. The Tides. 

50. The Sun as a Source of Terrestrial Energy. 



QUERIES FOR USE IN REVIEWS AND EXAMINATIONS. 315 



APPENDIX VI. 



475. QUERIES FOR USE IN REVIEWS AND EXAMINATIONS. 

The following questions are intended to embrace the most important 
topics treated in Chapters I. to XIV. 

1. Name the principal classes of celestial objects. 

2. What is the celestial sphere, as defined by mathematical astronomers? 

3. Define the celestial poles, the celestial equator, the zenith, the nadir, and 
the plane of the horizon. 

4. Explain the chief difference between reflecting and refracting telescopes. 

5. Explain the function of the object-glass, and of the eyepiece of a tele- 
scope. 

6. Give the meanings of the terms reflection, refraction, and dispersion of 
light. 

7. Explain how a telescope is made to follow any star by means of an equa- 
torial mounting. 

8. Give some hints as to the method of using a small telescope. 

9. State the distance, diameter, and rotation time of the sun. 

10. Describe the appearance of a sun spot, and tell of the periodicity and 
cause of these strange objects. 

11. Describe the photosphere, chromosphere, prominences, and corona. 

12. Describe the construction of a spectroscope. 

13. Give the laws of spectrum analysis. 

14. Give some illustrations of the distance, light, and heat of the sun. 

15. Explain the "contraction theory" of the maintenance of the sun's heat. 

16. Explain how the earth's diameter is found. 

17. Why does not the plumb-line always point toward the earth's centre ? 

18. Tell how to draw an ellipse : define foci, major axis, minor axis, perihelion, 
and aphelion. 

19. Define ecliptic, equinoxes, and solstices. 

20. Explain why the days are long in summer, and short in winter, in middle 
latitudes. 

21. Why is the sun continuously above the horizon, at the north pole, for 
six successive months ? 

22. Explain the two principal causes of the changes of the seasons. 

23. Give the cause of the precession of the equinoxes, and draw a diagram 
showing the movement of the north celestial pole due to precession. 



316 DESCRIPTIVE ASTRONOMY. 

24. Show how the aberration of light affects the direction in which a tele- 
scope points. 

25. State the cause of refraction, and its effect on the apparent place of a 
star. 

26. Define the sidereal year and the tropical year, and show why one is longer 
than the other. 

27. State the principle according to which leap years are determined in the 
Gregorian calendar. 

28. Give accurate definitions of the plane of the meridian of any place, and 
of the latitude and longitude of the place. 

29. Define celestial meridian (of any point on the earth), prime vertical, alti- 
tude, and azimuth. 

30. Define hour circle, right ascension, declination, north polar distance, and 
hour angle. 

31. What is meant by the horizontal parallax of a celestial object ? 

32. What is the difference between a mean solar day and an apparent solar 
day ? 

33. State the causes of the unequal lengths of apparent solar days. 

34 What is the distinction between a civil day and an astronomical day? 

35. Explain the relation between sidereal time and right ascension. 

36. Describe a meridian circle. 

37. Tell how to find the error of a clock by observing stars with a meridian 
circle. 

38. Prove that the altitude of the pole equals the latitude of the place of 
observation. 

39. Give the demonstration for finding the latitude of a place by observing 
altitudes of the pole star. 

40. How is the longitude between two cities found by aid of the telegraph? 

41. How is the position of a ship found at sea? 

42. What is meant by the sidereal and synodic periods of the moon ? 

43. State the causes of the libration of the moon. 

44. Describe and explain the phases of the moon. 

45. Describe the general characteristics of the lunar surface. 

46. State some reasons for believing that the lunar atmosphere is extremely 
rare. 

47. Give an explanation of the disappearance of the air and water which the 
moon may have possessed in the past. 

48. Explain the cause of the coldness of the lunar surface. 

49. State a few superstitions about the moon, and tell of its real worth to man. 

50. Draw a diagram and make plain the meanings of the umbra and penumbra 
of the shadow of the earth or of the moon. 

51. Describe the successive appearances of the moon during a total lunar 
eclipse. 

52. Give the reasons why solar eclipses are sometimes partial, sometimes total, 
and sometimes annular. 



QUERIES FOR USE IN REVIEWS AND EXAMINATIONS. 317 

53. Describe the phenomena of a total solar eclipse. 

54. What observations are made by astronomers at the time of a total solar 
eclipse ? 

55. State Newton's law of gravitation. 

56. State Kepler's laws of planetary motion. 

57. Define superior and inferior planets. 

58. Define the conjunction, opposition, elongation, and quadrature of a planet. 

59. Explain why a superior planet retrogrades. 

60. State the diameters, distances (from the sun), times of revolution, and 
times of rotation, of Mercury, Venus, the Earth, and Mars. 

61. Tell of the telescopic appearance and physical condition of Mercury. 

62. Tell of the telescopic appearance and physical condition of Venus. 

63. Describe a transit of Venus, and tell the special use that astronomers 
have made of such transits. 

64. Tell about the polar caps, seas, continents, clouds, and atmosphere of 
Mars. 

65. Describe the canals and satellites of Mars. 

66. State and comment on Bode's law. 

67. Recount the circumstances of the discovery of the first minor planet, and 
the computation of its orbit. 

68. Describe the present methods of discovering and keeping track of the 
asteroids. 

69. Give theories of the origin of the asteroids. 

70. State the diameters, distances (from the sun), times of revolution, and 
times of rotation, of Jupiter, Saturn, Uranus, and Neptune. 

71. Describe the telescopic appearance of Jupiter. 

72. Tell of the. atmosphere, light, heat, and physical condition of Jupiter. 

J2>- Describe the satellites of Jupiter : also explain their eclipses, occultations, 
and transits. 

74. Show how the velocity of light was discovered by observations of Jupiter's 
moons. 

1$. Describe the telescopic appearance of Saturn. 

76. Narrate the history of the discovery of Saturn's rings, and describe their 
changes of appearance. 

JJ. Discuss the structure and stability of Saturn's ring system. 

78. Tell about Saturn's satellites, and the physical condition of the planet. 

79. Narrate the history of the discovery of Uranus. 

80. Tell of the telescopic appearance, the satellites, and the ohysical condi- 
tion of Uranus. 

81. Tell the history of the discovery of Neptune. 

82. Tell of the telescopic appearance, the satellite, and the physical condition 
of Neptune. 

83. Describe the present methods of searching for comets. 

84. Tell how comets are designated. 

35. Name and describe the parts of a comet. 



318 DESCRIPTIVE ASTRONOMY. 

86. Name the forms of the orbits of comets and state the significance of 
these forms. 

8j. Tell about groups and planet's families of comets. 

88. Describe the changes in the appearance of a comet as it approaches 
the sun. 

89. State the supposed constitution of the head and nucleus of a comet. 

90. Describe the evolution of a comet's tail, and the three types of tails. 

91. What causes give to comets their brightness? 

92. Why have comets been dreaded, and what occasion is there for dread ? 

93. Narrate the histories of three remarkable comets. 

94. Describe the two classes of meteors. 

95. Give an account of some noted meteorite. 

96. Explain carefully the effect of the swift rush of a meteorite through 
the air. 

97. What are meteorites composed of ? 

98. State the theories of the origin of meteorites. 

99. Tell how to observe the path of a meteorite. 

100. Explain why more shooting stars are seen in the early morning than in 
the evening. 

101. Tell about the velocities and masses of shooting stars. 

102. Define and explain the radiant of a meteoric shower. 

103. Describe a great meteoric shower. 

104. Give the supposed history of the Leonids. - 

105. Tell the interesting facts about the Bielids. 

106. State the relation between comets and meteors. 

107. Describe the zodiacal light, and give a theory of its cause. 

108. Tell about the number of fixed stars visible with different means, and 
explain their scintillation. 

109. Describe the appearance of the Milky Way. 

no. Give the history of the naming of the constellations now recognized, 

in. State the methods of naming individual stars. 

112. How is the brightness of a star denoted? 

1 13. How are the stars distributed in the heavens? 

114. Tell about star clusters. 

115. What are the stars, and how large are they ? 

116. Define the parallax of a fixed star. 

117. Explain how the distances of the fixed stars are found. 

118. State the supposed causes of the various colors of stars. 

119. Describe the types of stellar spectra. 

120. Give the theories as to the form of the visible stellar universe. 

121. What is meant by the " proper motions " of the stars ? 

122. How are the velocities of stars in the line of sight found ? 

123. How has the direction of the sun's motion in space been determined ? 

124. What evidence is there bearing on the question whether the stellar uni- 
verse is an orderly system ? 






QUERIES FOR USE IN REVIEWS AND EXAMINATIONS. 319 

125. What is the method of naming double stars ? 

126. State the distinction between physical and optical doubles. 

127. How does the spectroscope show that some stars are binaries, when sim- 
ple visual observations with a telescope would never reveal the fact? 

128. Tell the story of the discovery of the companion of Sirius. 

129. Describe two multiple stars. 

130. Define a variable star, and a periodic variable. 

131. State the five classes of variable stars. 

132. Give an account of Tycho's temporary star. 

133. Describe the variations of Algol, and their cause. 

134. Tell the story of Nova Aurigae. 

135. How are variables observed by astronomers ? 

136. State the supposed causes of stellar variability. 

137. Tell the different forms of nebulae. 

138. What is the law of distribution of the nebulae over the face of the sky ? 

139. What kinds of spectra do nebulae give ? 

140. Describe the great nebula in Orion, or that in Andromeda. 

141. What are the Magellanic Clouds? 

142. What is Professor Holden's theory of the real form of spiral nebulae ? 

143. State the nebular hypothesis according to Laplace. 

144. What modifications of Laplace's theory have been made ? 

145. Give the testimony of the nebulae and of the stars to the truth of the 
nebular hypothesis. 

146. State the testimony to the truth of the nebular hypothesis given by the 
motions of the planets. 

147. What is the testimony of astronomy as to the future of the visible uni- 
verse ? 

148. Tell how to find the northern constellations on any evening by the aid 
of Map I. 

149. Tell how to find the southern constellations on any evening by the aid of 
Maps II. to V. 

150. Give some hints useful for learning and fixing in mind the constellations. 



320 DESCRIPTIVE ASTRONOMY. 



APPENDIX VII. 

476. LIST OF REFERENCE BOOKS. 

The following list of books upon Descriptive Astronomy is given to aid in 
the formation of a reference library. With such a wealth of good material 
to choose from, one ought not to go far astray. A generous selection of such 
books would be found very helpful. 

Ball. Atlas of Astronomy. D. Appleton & Co., Publishers. $4.00. 

There are 72 plates, 34 of which are devoted to star maps, on which all stars 
down to the sixth magnitude inclusive are shown with great distinctness. The 
very complete index map of the moon occupies several plates. Directions are 
given for locating the planets among the stars, up to 1902. The list of select tele- 
scopic objects contains exceptionally full descriptions of them. Many unique 
features commend the Atlas strongly to students and amateur observers. 

Ball. Great Astronomers. Isbister & Co., Publishers, pp. 372. 7s. 6d. 
In this sketchy book are pen pictures of 18 astronomers from Ptolemy on- 
ward. There is much chatty information, together with numerous illustrations of 
these famous men and their observatories. 

Ball. In Starry Realms. J. B. Lippincott Co.. Importers. pp. 364. 
$2.50. 

A series of finely written essays on interesting matters pertaining to the 
heavenly bodies. They are suited for supplementary reading in connection with 
any text-book on elementary astronomy. There are two non-astronomical chap- 
ters, one devoted to the eruption of Krakatoa in August, 1883, and the other to 
the relation of Darwinism to various sciences. 

Ball. In the High Heavens. J. B. Lippincott Co., Publishers. pp. 
383. $2.50. 
A readable book on various astronomical topics of interest. 

Ball. Starland. Ginn & Co., Publishers, pp. 376. $1.00. 

A charming book for boys and girls, and for " children of a larger growth," 
who have a desire to refresh their knowledge of astronomy. Gladstone read it 
with pleasure. 

Ball. The Story of the Heavens. Cassell & Co., Publishers, pp. 536. 
$5.00. 
A popular astronomy, written in a delightful style : the chapter on the tides is 



LIST OF REFERENCE BOOKS. 32 1 

especially noteworthy : it explains in a simple manner Prof. G. H. Darwin's 
theory of tidal evolution, as illustrated in the case of the moon and the earth. 

Blake. Astronomical Myths. Macmillan & Co., Publishers, pp.431. $2.00. 
This is based on Flammarion's " History of the Heavens." It treats of the 
beginnings of astronomy, and of the many theories held in ancient and mediaeval 
times concerning the structure of the heavens and of the earth. 

Boeddicker. The Milky Way. Longmans, Green & Co., Publishers. 
$10.00. 
Dr. Boeddicker has prepared four plates of the Milky Way, each 18 X 23 
inches, showing that wonderful aggregation of suns, as it appears to the keen eye 
of a painstaking observer. The Via Lactea is delineated from the north pole to 
io° of south declination. The exceeding complexity of its structure is a revela- 
tion to one who has never made a careful study of it. 

Brewster. The Martyrs of Science. Chatto and Windus, Publishers, 
pp. 248. $1.80. 

Brief and interesting biographies of Galileo, Tycho, and Kepler, by Sir David 
Brewster. 

Chambers. Handbook of Descriptive and Practical Astronomy. Fourth 
Edition. The Clarendon Press, Oxford, Publishers. 3 vols. pp. 16 18. 
$14.00. 

A miniature encyclopaedia in its field; valuable to amateur astronomers; a 
good reference book for teachers and advanced scholars. 

Chambers. Pictorial Astronomy for General Readers. Whittaker & Co., 
Publishers, pp. 268. $1.25. 
The descriptive matter is good, but some of the cuts are atrociously executed. 
There are lists of the most interesting celestial objects suitable for observation 
with a three-inch telescope. 

Clerke. History of Astronomy during the Nineteenth Century. Adam and 
Charles Black, Publishers. Third Edition, revised and enlarged, pp. 
500. $4.00. 
An excellent work, written in an interesting style. Those teachers and older 

scholars who take special interest in astronomy will find its perusal delightful 

and helpful. 

Clerke. The Herschels and Modern Astronomy. Macmillan & Co.. Pub- 
lishers, pp. 224. $1.25. 
A delightful account of the lives and scientific activities of Sir William 
Herschel, his devoted sister Caroline, and his son Sir John. The ardent purposes 
and high ideals of the subjects of the sketch are well set forth. 



32 2 DESCRIPTIVE ASTRONOMY. 

Clerke. The System of the Stars. Longmans, Green & Co., Publishers, 
pp. 440. S~ . 00. 

This is the most exhaustive work on the fixed stars in the English language : 
it is a useful book of reference. Sidereal astronomy is making rapid strides, 
which are well described : stress is laid upon the latest theories of the construc- 
tion of the sidereal universe. 

Colas. Celestial Planisphere. Poole Bros., Publishers. $3. 00. 

This is one of the best of planispheres. It consists of a movable disk, 19 
inches in diameter, attached by a pivot to a heavy rectangular piece of card- 
board which measures 18^X23 inches. Nearly all stars visible to the naked 
eye, down to 50 of south declination, and the chief nebulae, are depicted on it. 
It can, like all planispheres, be adjusted and held in such a way as to show the 
face of the sky at any moment ; the time when any star rises, sets, or culminates 
on a given day, can be ascertained from it. It is accompanied by a celestial 
handbook of no pages, in which, after a few pages of definitions, detailed de- 
scriptions of the constellations are given : the principal objects of interest in each 
are mentioned. The price of the handbook is S2.00. 

Colas. The Moon, a Map. Poole Bros., Publishers. 

The map is 20 inches in diameter, and is printed in colors : an index pamphlet 
of 24 pages by Prof. W. W. Payne accompanies it. Extremely satisfactory. 

Denning. Telescopic Work for Starlight Evenings. Taylor and Francis, 

Publishers, pp. 361. S2.00. 
The first three chapters are devoted to the telescope : the methods of testing 
and handling it are explained. The relative merits of refractors and reflectors 
are set forth. The remaining chapters are filled with descriptive matter about 
the heavenly bodies. Meteors and meteoric observations are treated quite fully, 
the author being a specialist in that line of work. 

Dreyer. Tycho Brahe, a Picture of Scientific Life and Work in the Six- 
teenth Century. Adam and Charles Black, Publishers, pp.405. S3. 50. 
A thoroughly reliable and readable account of the life and scientific surround- 
ings of one of the greatest of astronomers. 

Elger. The Moon. George Philip and Son, Publishers, pp. 173. 5s. 

This work is devoted to a description of the craters, seas. etc.. on the 
lunar landscape, and contains excellent maps. The best medium-priced lunar 
handbook. 

Frost. Astronomical Spectroscopy. Translated from the German of Dr. 
J. Schemer, and revised with the author's sanction. Ginn & Co., Pub- 
lishers, pp. 450. $5. 00. 
The most practical methods of spectroscopic observations are set forth in de- 
tail, and the knowledge thus far gained by means of astronomical spectroscopy 






LIST OF REFERENCE BOOKS. 323 

is admirably stated. The book is fairly entitled to be called indispensable to 
workers along spectroscopic lines. Teachers will find it useful as a work 
of reference. 

Gore. Flammarion's Popular Astronomy. Chatto and Windus, Publishers. 
pp. 679. 16 shillings. 
This is a translation from the French, the original having reached a sale of 
over 100,000 copies. The book is finely illustrated, and very popular in style. 

Gore, J. E. The Scenery of the Heavens. Roper and Drowley, Publishers. 
pp.320. $4.00. 
This work contains a general account of the heavenly bodies, together with 
lists and descriptions of the most interesting double stars, nebulae, and vari- 
able stars. 

Gore, J. E. The Visible Universe. Macmillan & Co., Publishers, pp. 340. 

fe-75- 

This work deals with the different theories of solar and stellar evolution, the 
ether, the constitution of matter, and the theories of the shape of the visible uni- 
verse, large space being given to the last subject. The elegance of the illustra- 
tions befits the excellence of the text. 

Gore, J. H. Geodesy. Houghton, Mifflin & Co., Publishers, pp. 218. 
$1.25. 
The author has given an historical sketch of the various important attempts 
to measure the magnitude and determine the form of the earth, from the 
earliest times. 

Kirkwood. The Asteroids. J. B. Lippincott Co., Publishers. pp. 60. 
$0.50. 
In addition to general descriptive matter, the author makes a special study of 
the distribution of the orbits of asteroids, giving reasons for the gaps which are 
found in them. 

Klein. Star Atlas. E. & J. B. Young & Co., Publishers, pp. 72, aside 
from maps and plates. $2.00. 

The maps are twelve in number, each measuring 12 X. 9 inches : they contain 
all stars, from the sixth magnitude upward, between the north pole and 30 of 
south declination. There is a 60-page list of interesting telescopic objects, with 
a good description of each one. This is probably the best low-priced atlas in 
the English language. 

Langley. The New Astronomy. Houghton, Mifflin & Co., Publishers, 
pp. 260. $5.00. 
The term " new astronomy " is used chiefly with reference to spectroscopic 
and photographic work. The book is very finely illustrated : the author's stylo is 



324 DESCRIPTIVE ASTRONOMY. 

so elegant that the reader's attention is closely held throughout. It would 
be hard to find an astronomical work more attractively written, or better 
illustrated. 

Lockyer. The Dawn of Astronomy. Macmillan & Co., Publishers, pp. 
432. S5. 00. 
This work contains a study of the mythology and temple worship of the 
ancient Egyptians, with special reference to their astronomical bearings. The 
book is elaborately illustrated, and is suitable for reference, rather than for gen- 
eral reading. 

Lockyer. The Meteoritic Hypothesis. Macmillan & Co., Publishers, pp. 

5 6 °- $5- 2 5- 
The sub-title is " A Statement of the Results of a Spectroscopic Inquiry into 
the Origin of Cosmical Systems." The book sets forth the theory that all 
celestial bodies are composed of meteors, more or less thickly crowded together, 
and gives in detail the observations and experiments on which the theory is 
based. The illustrations are fine : some portions of the text are of interest to 
non-astronomical readers. 

Lowell. Mars. Houghton, Mifflin & Co., Publishers, pp.217. $ 2 -5°- 

A popular account of Mars, with special reference to the question of the 
existence of intelligent beings on its surface : elegantly illustrated by full page 
plates. 

Mitchel. Ormsby Macknight Mitchel, Astronomer and General. Hough- 
ton, Mifflin & Co., Publishers, pp. 392. $2. 00. 
An admirable biography of a remarkable man, who built the Cincinnati Ob- 
servatory, raising the necessary funds by a popular subscription. Most of the 
subscriptions came from tradesmen and mechanics : many were payable in com- 
modities or labor. 

Nasmyth and Carpenter. The Moon. Scribner and Welford. pp. 213. 
$9.00. 
During more than thirty years the authors studied the moon's surface with 
powerful telescopes ; they made careful drawings, and then constructed accurate 
models of the lunar craters ; these were photographed. The book contains 25 
very fine plates. 

Newcomb. Popular Astronomy. School Edition. Harper and Brothers, 
Publishers, pp. 578. $1.30. 
An excellent presentation of the subject, written by one of the ablest of 
astronomers. 

Parker. Familiar Talks on Astronomy. A. C. McClurg & Co., Publishers, 
pp. 264. $1.00. 
These are such talks as a teacher might give to a class in elementary as- 



LIST OF REFERENCE BOOKS. 325 

tronomy, in addition to the regular class work. The last four chapters are 
about time and nautical astronomy, viewed from the standpoint of a practical 
navigator. 

Proctor. Half Hours with the Stars. G. P. Putnam's Sons, Publishers. 
pp. 38. $2.00. 
The book is in quarto form, and contains twelve large maps, showing the 
aspect of the heavens throughout the year. The text is devoted to explaining 
how to find the star groups. Very few stars fainter than the fourth magnitude 
are shown. The work is admirably adapted to the needs of a beginner, who 
wishes to become familiar with the constellations. 

Proctor and Ranyard. Old and New Astronomy. Longmans, Green & 
Co., Publishers, pp. 824. $12.00. 

This volume is published in the form of a quarto. Mr. Proctor considered it 
as, in a sense, the summing up of his numerous writings on astronomy. It was 
not completed at the time of his death, and Mr. Ranyard supplied the portion 
lacking. The work is a fitting crown to Mr. Proctor's long service in populariz- 
ing astronomy. Mr. Ranyard's careful editing and supplementary writing add 
much to its value. 

Servtss. Astronomy with an Opera-glass. D. Appleton & Co., Publishers, 
pp. 154. $1.50. 

Maps of the constellations are given, and directions for finding them : promi- 
nent celestial objects are described ; the author gives directions for choosing a 
good opera-glass, and shows that many pleasurable views of the heavenly bodies 
may be obtained by its aid. 

Thornton. Advanced Physiography. Longmans, Green & Co., Publishers. 
pp. 338. $1.40. 

One of the very best works on this subject: it is largely an elementary astron- 
omy, but about one fourth of the book is devoted to atmospheric and oceanic 
motions, magnetism, and the secular cooling of the earth. 

Webb. Celestial Objects for Common Telescopes. Fifth Edition, revised 
and greatly enlarged by T. E. Espin. 2 vols. Longmans, Green & 
Co., Publishers. $3.50, 
This is the most complete and authoritative book, in its field, in our language. 

Volume I. tells how to use a telescope for visual, photographic, and spectroscopic 

work, and contains chapters on the sun, moon, planets, comets, and meteors. 

Volume II. gives lists and full descriptions of the principal stars, clusters, and 

nebulae visible to us. 

Winchell. World-Life, or Comparative Geology. S. C. Griggs & Co., Pub- 
lishers, pp. 642. $2.50. 
The work is written from the standpoint of a geologist, and is chiefly devoted 



326 DESCRIPTIVE ASTRONOMY. 

to an elaborate account of the formation of the different planets, in accordance 
with the principles of the nebular hypothesis. There are also chapters on Plan- 
etary Decay, The Habitability of Other Worlds, and The Evolution of Cosmo- 
gonic Doctrine. 

Young. The Sun. D. Appleton & Co., Publishers, pp. 363. $2.00. 

The best work on this subject, in English : suitable for reference and for col- 
lateral reading. The author is one of the most distinguished students of the 
sun. 

Young. General Astronomy. Ginn & Co., Publishers, pp. 551. $2.50. 

A text-book for advanced collegiate students, replete with accurate informa- 
tion : easily the first of its kind. 



INDEX. 



All numerical references are to Sections. 



Abbe, Professor, 39. 

Aberration of light, 1 16. 

Achromatic Object-glass, 39. 

Achromatism, almost perfect, 39. 

Adams, J. C, computes orbit of Neptune, 271. 

Aerolite. (See Meteorites.) 

Age of the sun, 87. 

Algol, diameter of, 348 ; variations of, 379 ; position, 449. 

Alpha Centauri, 349, 350. 

Alphabet of the Greek language, 405. 

Altitude defined, 121. 

Anderson, Thos. D., discovers Nova Aurigae, 381. 

Andromeda, nebula in, 389, 398, 407 ; constellation, 407. 

Andromedes, 330. 

Annular eclipses, 175, 176; nebulas, 385. 

Aphelion, explained, 96. 

Apparent Motion, of the stars daily, 13; of the celestial sphere, 17 ; of the 

sun, 125. 
Aquarius, 408. 
Aquila, 409. 
Argo Navis, 410. 
Aries, a sign of the zodiac, 100; the sun enters, 112; non-coincidence of sign 

and constellation, 112; the constellation, 41 1. 
Aspects of the planets, 188. 
Asteroids, discovery, 224-227 ; orbits, distances, periods, 228 ; designations, 

229; number and sizes, 230; atmosphere and surface gravity, 231; 

origin, 232. 
Astrsea discovered, 226. 
Astronomical Constants, 471. 
Astronomy, landmarks in the history of, 473. 

Atmosphere of the moon, 162 and 164; of the planets (see each planet). 
Attraction of gravitation, 183, 368. 
Auriga, 412. 
Azimuth, defined, 12T. 



328 INDEX. 

Bailey, S. I., discovers variables in clusters, 373. 

Ball. Sir Robert, accounts for the rarity of the atmosphere of Mars, 217. 

Barnard, E. E., drawing of Mars, 213; observes changes on Mars, 215 ; ob- 
serves Jupiter's moons, 249; discovers the fifth satellite of Jupiter. 251; 
observes a moon of Saturn in the shadow of the dark ring, 260 : discovers a 
comet by photography, 280 ; photographs Brooks's comet, 307 ; observes the 
Gegenschein, 324, note ; first photographed the star clouds of the Milky 
Way, 338. 

Berliner Jahrbuch, 228. 

Beta Lyrae, 378, 

Bethlehem, star of. 201. 

Biela discovers a comet, 303. 

Bielids, 330. 

Binary Stars. 368: spectroscopic, 369. 

Black Drop, 204. 

Bode's Law, 223. 

Books for reference, 476. 

Bootes, 413. 

Brashear. J. A., optician, 39. 

Bredichin, investigates the forms of comets* tails, 295. 

Brooks, Wm. R., discovers comet c 1893, 307. 

Bruce photographic telescope, 277, 345. 

Burnham, S. W., 366. 

Caesar, Julius, his calendar, 114. 

Calendar, the Julian, 114; the Gregorian, 115. 

Camelopardus. 414. 

Campbell, W. W., observes Jupiter's moons with the Lick telescope, 249; also 

observes Nova Aurigae, 381. 
Canals of Mars, 218, 219. 
Cancer. 415. 
Canes Venatici, 416. 
Canis Major. 417. 
Canis Minor, 418. 
Capricornus, 419. 
Capture of comets, 287. 
Carrington, observation of the sun, 60. 
Cassini discovers the main division of Saturn's rings, 258. 
Cassiopeia, 1 1 , 406, 420. 
Catalogues of stars, 344. 

Celestial Sphere, defined, 15, 16; daily motion of, 17. 
Centaurus, 421. 
Cepheus, 422. 
Ceres discovered, 224, 225. 
Cetus. 423. 






INDEX. 329 

Chandler, S. C, catalogue of variables, 373 ; period of Algol, 379 ; variation of 

latitude, 95. 
Changes on the moon, 161. 
Chinese record of a meteor, 314. 
Chromosphere, described, 76. 
Chronograph, 139, 140. 
Chronometer, used on shipboard, 143. 
Civil Day, 129. 

Clairaut, investigates the orbit of Halley's comet, 301. 

Clark, Alvan, 39 ; his son, Alvan G., discovers the companion of Sirius, 370. 
Classification, of the planets, 187; of stellar spectra, 353; of variable stars, 

374- 

Clocks, 135; their errors determined, 138. 

Clusters of stars, 347 ; list of, 469. 

Colored stars, list of, 469. 

Columba, 424. 

Coma of a comet, 283 ; first appearance of, 289. 

Coma Berenices, 425. 

Comet, of 1861, 5, 300; of 1843, 291; 1889 V (Brooks), 292; Halley's, 299, 
301; of 1528, 299; Biela's, 300, 303, 306, 330, 332; Encke's, 302; Holmes's, 
303, 306; of 1882, 304; Swift's, of 1892, 305; c 1893 (Brooks), 307; Tem- 
pers, 332. 

Comets, derivation, 5; in general, 279; discovery, 280; number and designation, 
281 ; brightness and visibility, 282; parts of, 283 ; forms of orbits, 284 ; sig- 
nificance of forms of orbits, 285; groups, 286; planetary families, 287; 
changes in orbits, 288 ; changes of appearance, 289 ; jets and envelopes, 290; 
tails, 291; companion comets, 292; constitution, 293; evolution of the tail, 
294; types of tails, 295 ; mass and density, 296; light and spectra, 297; fate, 
298 ; superstitions, 299 : collisions, 300. 

Common, A. A., his large reflector, 43. 

Conic Sections, 284. 

Conjunction of planets, t88. 

Constants, list of, 471. 

Constellations, defined, 1 ; names of, 9 ; how to find, 10, 11 ; history of, 340. 

Constitution of the sun, 88. 

Contraction of the sun, 86. 

Copernicus publishes his great work in 1543, 473. 

Corona, appearance of, 80; Schaeberle's thec-y of, 81 ; nature of, 82. 

Corona Borealis, 426. 

Coronium, 82. 

Corvus, 427. 

Crater, 428. 

Craters on the moon, 157, 158. 

Crescent moon, 150. 

Cygnus, 429. 



330 INDEX. 

Data, planetary, 472 ; historical, 473. 

Date, change of, 133. 

Day, length of, 103-105; mean and apparent solar, 125-127; sidereal, 128; civil 

and astronomical, 129. 
Declination, denned, 122. 
Deimos, 221. 
Delphinus, 430. 
Designation, of asteroids, 229; of comets, 281; of stars, 341; of double stars, 

366; of variable stars, 373. 
Dimensions, of comets' tails, 291 ; of stars, 348. 

Directions for finding an object by means of an equatorial telescope, 468. 
Disk of a star, 348. 

Dispersion of light, 37; corrected, 38. 
Displacement of spectral lines, 360. 
Distance, of the sun, 49; determined by parallax, 123, 351 ; of the moon, 145 ; 

of the stars, 349; of the nebulae, 386; of each planet, 472. 
Distribution, of the stars, 346 ; of the nebulae, 386. 
Double Stars, appearance, 364, 365 ; number and nomenclature, 366 ; optical, 

367; physical, 368; spectroscopic, 369; Sirius, 370; planetary systems, 371 ; 

evolution of, 397 ; list of, 469. 
Draco, 431. 

Draper, J. W., in 1840, first photographed the moon, 473. 
Dumb-bell Nebula, 391. 
Duration of human life on the earth, 87 ; of eclipses, 179. 

Earth, dimensions and shape, 90; diameter, how measured, 92; latitude and 
longitude on it, 93, 94 ; variation of latitude, 95 ; the orbit, 96 ; the ecliptic, 
97; the equinoxes, 98; the solstices, 99; zodiac, 100; length of the day, 
103-105; midnight sun, 106; seasons, 107,108; equatorial ring, 109; pre- 
cession explained, 109-111 ; effects of precession, 112; different kinds of 
years, 113; Julian calendar, 114; Gregorian calendar, 115; aberration of 
light, 116; atmospheric refraction, 117; twilight, 118. 

Earth Shine on the moon, 151. 

Eclipses, the one of April 16, 1893, 81 ; of the moon, 170-172; cause of solar, 
173; varieties of solar, 175; partial and annular solar, 176; total solar, 177, 
178; duration and number of, 179; of Jupiter's satellites, 246. 

Ecliptic, defined, 97; fixity of, 102. 

Ellipse, described, 96 ; a conic section, 284; changed into a parabola or hyper- 
bola, 288. 

Elongation of a planet, 188. 

Encke discovers a division in Saturn's rings, 258. 

Envelopes of comets, 290. 

Ephemerides of asteroids, 228. 

Epsilon Lyrae, 372. 

Equator, Celestial, denned, 20; fixity of, 102. 



INDEX. 331 

Equatorial Mounting, explained, 44-46. 

Equinoxes, denned, 98; precession of, explained, 109-112. 

Equuleus, 432. 

Eridanus, 433. 

Essays, topics for, 474. 

Evening Star, denned, 191 ; Venus, 201. 

Evolution of double stars, 397. 

Eyepieces, their action, 33; achromatic, 40; negative and positive, 40. 

Examinations, queries for, 475. 

Faculee, 54. 

Families of comets, 287. 

Galaxy, 338. 

Galileo, discovers the phases of Venus, 205; thinks Saturn triform, 257; notices 

the vibrations of a pendulum in 1583,473; makes a telescope and various 

discoveries with it in 1 609-1 61 3, 473. 
Galle discovers Neptune, 271. 
Gauss computes the orbit of Ceres, 225. 
Gegenschein, 334. 
Gemini, 434. 
Gibbous moon, 150. 

Gravitation, law of, 183; universal, 368. 
Great Circles defined, 120. 
Great Dipper, 20, 359, 369. 

Greek Alphabet, used in naming stars, 341 ; given, 405. 
Gregorian Calendar, 115. 
Groups of stars, 359. 

Habitability of Mars, 222. 

Hale, Geo. E., observes a solar disturbance, 61. 

Hall, A., discovers the moons of Mars, 221 ; monograph on the moons of Mars, 
222, note; determines rotation of Saturn, 254. 

Halley, his method of observing a transit of Venus, 204; his comet, 299, 301 ; 
in 1705 predicted its return, 473 ; in 1731 invented the sextant, 473. 

Heat, of the sun, 84; produced by contraction, 86; of the moon, 165. 

Hercules, 435. 

Herschel, Caroline, 265. 

Herschel, Sir John, makes a prediction about Neptune, 271; star-gauges 
of, 346. 

Herschel, Sir William, discovers Uranus, 265; star-gauges, 346; in 1S03 pub- 
lishes his discovery of binary stars, 473. 

Hesperus, 201. 

History of astronomy, 473. 

Hodgson, observation of the sun, 60. 



S3 2 INDEX. 

Holden. E. S., opinion of the polar caps of Mars. 213 : rinds motion in the trifid 
nebula, 3S7 : investigates spiral nebulae. 392. 

Holmes discovers a comet, 306. 

Horizon defined. 21. 121. 

Horizontal parallax. 123. 

Hour Angle, denned. 122: of a star at any instant. 466. 

Hour Circles defined. 122. 

Huyghens. discovers the rings of Saturn in 1665. 257. 473 : made the first pen- 
dulum clock in 1656. 473. 

Hydra. 436. 

Hyperbola, a conic section. 2S4 ; a comet"s orbit. 2S5. 

Image, formation of. 32. 
Inertia. 1S4. 
Intra-Mercurial planets. 178. 

Japetus, 263. 

Jena glass. 39. 

Jets from comets. 290. 

Josephus mentions a comet. 299. 

Julian Calendar. 114. 

Juno discovered. 226. 

Jupiter, occulted by the moon. 162 : may have disrupted the asteroid ring. 232 ; 
distance and diameter. 235: revolution and rotation. 236: appearance. 237- 
241: satellites visible with the naked eye. 237: belts. 23S. 239: great red 
spot. 240 : other spots. 239. 241 : atmosphere and spectrum, 242 : light and 
heat. 243 : physical condition. 244 : the major satellites, 245 : eclipses, occul- 
tations, and transits of the satellites. 246-248 : markings and rotation of the 
satellites. 249. 250: the fifth satellite. 251: observations of the moons give 
the velocity of light, 252 ; gathers a family of comets. 2S7 : disturbs Brooks's 
comet. 292. 

Kapteyn. J. C, investigates the form of the stellar system, 2>37- 

Keeler. J. E.. discovers a division in Saturn's rings. 258 : determines their struc- 
ture, 261 : determines the motion of the nebula in Orion. 390. 

Kepler, his speculations on lunar craters. 157 : his laws. 1S5 ; guesses the number 
of moons of the planets, 222. note: his opinion about comets, 281 ; publishes 
his first and second laws in 1609, 473 ; discovers his third law in 161 8, 473. 

Kirchhoffs Laws. 73. 

Krakatoa. eruption of, 324. 

Lacerta. 437. 

Landmarks in the history of astronomy. 473. 

Langley. S. P.. estimate of work done by the sun's heat. 84 : observations of 

Mars. 215. 
Laplace suggests a name for Uranus. 265 : his nebular hypothesis. 395. 396. 



index. 333 

Latitude, of terrestrial points, 93, 94 ; variation of, 95 ; method of determining, 

141 ; of a ship, 143. 
Leap Year, 115. 
Lenses, 30. 
Leo, 438. 
Leo Minor, 439. 
Leonids, 327-329. 
Lepus, 440. 

Leverrier computes the orbit of Neptune, 271. 
Libra, 441. 

Librations of the moon, 149. 
Lick telescope, 48. 
Life on the moon, 167. 

Light, of the sun, 83 ; velocity of, 83 ; of the moon, 165. 
Light Ratio, 343. 
Light Year, 349. 
Lists of telescopic objects, 469. 
Local Time, 130. 

Lockyer, J. N., his theory of variables, 382. 
Longitude, of terrestrial points, 93, 94 ; determined by telegraph, 142 ; of a ship, 

143- 
Lowell, Percival, 219. 
Lowell Observatory, 218, 221. 
Lupus, 442. 
Lynx, 443. 
Lyra, 444. 

Magellanic Clouds, 393. 

Magnetic Storms, 65-68. 

Magnifying Power of eyepieces, 36. 

Magnitudes of stars, 1, 8, 342, 343. 

Maps explained, 8. 

Mars, distance and diameter, 209 ; revolution and rotation, 210 ; appearance, 211, 

212; phases, 212; polar caps, 213 ; seas, 214; continents and islands, 215 ; 

clouds, 216; atmosphere, 217 ; canals, 218,219; colors, 220; satellites, 221; 

habitability, 222; Hall's monograph on the moons, 222, note. 
Mean Solar Time, 130. 
Mercury, distance and diameter, 195 ; revolution and rotation, 196; transits, 197'. 

appearance and phases, 198,199; physical condition, 200 ; perturbs Encke's 

comet, 302; transit first observed in 1631, 473. 
Meridian, terrestrial, defined, 94; celestial, defined, 121. 

Meridian Circle, described, 136 ; used to determine time, 137-138 : used to deter- 
mine latitude, 141. 
Meteorites, past appearances, 309 ; Ensisheim meteorite, 310; Kiowa County. 

Kansas, meteorite, 312 ; in flight, 313 ; path and velocity, 314 ; light and heat, 



334 INDEX. 

315,316; meteoric stones, 317 ; meteoric iron. 318 : elements found in, 319; 
Canyon Diablo, 319; origin, 320: observation of, 321. 

Meteors, defined, 6 ; two classes. 308 : a detonating, 311: path and velocity, 314: 
light and heat, 315, 316; meteoric stones, 317 ; meteoric iron, 318; elements 
found in, 319; origin, 320; observation of, 321. 

Midnight Sun, 106. 

Milky Way, 338 : tree-like structures in, 339 ; shape, 356. 

Minor Planets. [See Asteroids.) 

Mira Ceti, 376. 

Mizar, 369. 

Monoceros. 445. 

Month, sidereal and synodic. 146. 

Moon, distance, 145; diameter, 145: orbit, 145 : nodes, 145; periods, sidereal 
and synodic, 146: meridian passage, 147: rotation. 148: librations, 149; 
phases, 150 ; earth shine, 151 : occultations, 152 ; appearance to the naked eye r 
153: telescopic appearance, 154; topography, 155: the plains, 156; craters,. 
157, 158: mountains, 159; rills, clefts, and rays, 160; changes, 161: atmos- 
phere, 162: spectrum, 162: water, 163, 164: light and heat, 165; tempera- 
ture. 166; life, 167: effect on the weather, 168' worth to man, 169; eclipses, 
170-172; mountains visible in a solar eclipse, 176; duration and number 
of eclipses, 179; origin of its features. 400. 

Morning Star, defined. 191 ; Venus, 201. 

Motion, daily, of the heavens, 13, 17. 

Mountains, lunar, 1 59 : terrestrial. 400. 

Mount Hamilton, clouds visible there, 238. 

Mounting, equatorial, 44-46. 

Multiple Stars, 372. 

Nadir defined. 23. 

Nebulae, defined, 7 : various forms, 385 : number, distance, and grouping, 386 * y 
sizes and changes of appearance, 387 : spectra, 388 : nebula in Andromeda, 
389 : nebula in Orion. 390 : other notable. 391 : real form of spiral, 392 ; Ma- 
gellanic Clouds. 393 : the nebular hypothesis. 394-404: list of, 469. 

Nebular Hypothesis, general statement, 394 ; Laplace's theory and its modifica- 
tions. 395, 396 : evolution of double stars. 397: testimony of the nebulas, 
stars, earth, moon, planetary systems, and sun, 398-402; its probable truths 
403 : future of the universe, 404. 

Negative Eyepieces. 40. 

Neptune, does not conform to Bode's law, 223: discovery, 271: distance and 
diameter. 272 : revolution and rotation, 273 : appearance. 274 ; satellite, 275 ; 
physical condition, 276: captures comets. 287. 

Newton. Sir Isaac, law of gravitation, 1S3 : published the Principia in 1687,473. 

Newtonian telescope, 41. 

Nodes of the moon's orbit, 145. 

Nordenskiold finds supposed dust of shooting stars, 324. 



index. 335 

North Polar Distance defined, 122. 
Nova Aurigae, 381. 

Nucleus of a comet, 283 ; changes of, 289. 

Number, of eclipses in a year, 179; of asteroids, 230; of comets, 281 ; of shoot- 
ing stars, 322 ; of fixed stars, 336; of double stars, 366; of nebulae, 386. 

Object-glass, use of a large one, 35 ; achromatic, 39. 

Oblate Spheroid, 90. 

Obliquity of the ecliptic defined, 97. 

Occultations, by the moon, 152 ; of Jupiter's satellites, 247. 

Omicron Ceti, 376. 

Ophiuchus, 446. 

Opposition of a planet, 188. 

Orbits of the planets, 181. 

Origin, of the asteroids, 232 ; of meteors, 320 

Orion, nebula in, 390, 447 ; constellation, 447. 

Pallas discovered, 226. 

Parabola, a conic section, 284; a comet'-s orbit, 285. 

Parallax, defined, 123; horizontal, 123; equatorial horizontal, 123; stellar, 350,. 

351 ; of nebulae, 386. 
Pare describes a comet, 299. 
Pegasus, 448. 

Pendulum, compensation of, 135. 
Penumbra, of a sun spot, 55; of a shadow, 170. 
Perihelion explained, 96. 
Periodic Comet defined, 286. 

Periods, sidereal and synodic, of the moon, 146; of planets, 192. 
Perseids, 326. 
Perseus, 449. 
Perturbations, 186. 
Phases of the moon, 150. 
Phobos, 221. 
Phosphorus, 201. 

Photographic Congress in 1887, 473. 
Photographs, of faculae, 54 ; of a solar disturbance, 61 ; of the corona, 81 ; of 

the moon, 161 ; of asteroids, 227; of an ultra-Neptunian planet, 277; of 

comets, 280, 305, 307 ; of shooting stars, 333 ; of the Milky Way, 338 : 

of stars, 345 ; of clusters, 347; of nebulae, 3S9-391. 
Photosphere of the sun, 52. 
Piazzi discovers the first asteroid, 224. 
Pickering, E. C, estimates diameters of the moons of Mars. 221 ; plans star 

charting,. 345 ; views of stellar spectra, 354; announces the duplicity of 

Mizar, 369. 
Pickering, W. H., opinion of the water area of Mars, 214; observations of 



336 INDEX. 

the canals of Mars, 218; observations of Jupiter's belts, 239; observes Jupi- 
ter's satellites, 250. 

Pisces, 450. 

Piscis Australis, 451. 

Planetary Data, 472. 

Planetary nebulae, 385, 398. 

Planets, denned, 2; orbits, 181; motion, 182-186; two classes, 187 ; aspects, 
188; apparent movements, 189-190; evening and morning stars, 191; peri- 
ods, sidereal and synodic, 192; two groups of, 194; Mercury, 195-200; 
Venus, 201-208 ; Mars, 209-222 ; minor planets, 223-232: Jupiter, 235-252; 
Saturn, 253-264; Uranus, 265-270; Neptune, 271-276; beyond Neptune, 
277; testimony for the nebular hypothesis, 401 ; data concerning, 472. 

Pleiades, 347; proper motions of, 359; nebulous matter in, 386. 

Plumb-line, direction of, 91. 

Pointers, 18. 

Pole, location of the north celestial, 18 ; fixity of, 101 ; precessional motion of, 112. 

Positive Eyepieces, 40. 

Prcesepe, 347. 

Precession, explained, 109-111 ; effects of, 112. 

Prime Vertical defined, 121. 

Prominences, Solar, quiescent and eruptive, 77 ; seen with the spectroscope, 
78 ; associated with magnetic storms, 79. 

Proper Motion of the stars, 358, 359. 

Ptolemy, revises the scheme of constellations, 340 ; his work, 473. 

Pupin, M. I., electrical discharges, 82. 

Pythagoras, theory of the daily motion of the sky, 13 ; his teaching, 473. 

Quadrature of a planet, 188. 

Queries for reviews and examinations, 475. 

Radiant, 325. 

Radius Vector defined, 181. 
Ranyard, A. C, his theory about clusters, 347. 
Red Spot on Jupiter, 240. 
Reference Books, list of, 476. 

Reflecting Telescope, 25 ; explained, 41 ; Newtonian, 41 ; comparison with a re- 
fractor, 42; noted ones, 43. 
Refracting Telescope, 25, 34; comparison with reflector, 42 ; invention of, 473. 
Refraction by a prism, 28; atmospheric, 117; effects in total lunar eclipse, 172. 
Reticle, 136. 

Retrograde Motion of a planet, 190, 
Reviews, queries for, 475. 
Rice Grains, in the sun, 53. 
Right Ascension defined, 122. 
Ring Nebula, 391, 444. 



index. 337 

Rings of Saturn, 257-262. 

Roberts, Isaac, photographs of nebulae, 389, 391. 

Roemer determines the velocity of light in 1675, 2 5 2 j 473- 

Rosse, Lord, his great reflector, 43. 

Rotation of the sun, 58. 

Sagitta, 452, 

Sagittarius, 453. 

Satellites, attending upon stars, 371 ; of the planets, 472. 

Saturn, distance and diameter, 253 ; revolution and rotation, 254 ; appearance, 
255, 256 ; discovery of the rings, 257 ; divisions and dimensions of the rings, 
258; disappearance of the rings, 259; the dark ring, 260; structure of the 
rings, 261 ; stability of the rings, 262 ; the satellites, 263 ; physical condi- 
tion, 264. 

Scheeberle, J. M., theory of the corona, 81 ; theory of the markings on Mars, 
215; observes Jupiter's moons, 249. 

Schiaparelli, determines rotation time of Mercury, 196 ; determines rotation time 
of Venus, 203 ; discovers canals of Mars, 218. 

Schott, Doctor, 39. 

Schwabe, observations of the sun, 59. 

Scintillation of the stars, 337. 

Scorpio, 454. 

Screen, used in solar observations, 51. 

Sculptor, 455. 

Scutum, 456. 

Seasons, in middle latitudes, 107; at the equator, 108. 

Secchi classifies stellar spectra, 353. 

Secondary Circles defined, 120. 

See, T. J. J., investigates the origin of double stars, 397. 

Serpens, 457. 

Sextans, 458. 

Sextant, 143. 

Shadows, of the earth and moon, 170, 174; shadow of the moon visible, 177. 

Ship, the position of, determined, 143. 

Shooting Stars, described, 308 ; numbers, 322 ; paths and velocity, 323; masses 
and constituents, 324; radiant, 325 ; the August shower, 326; the November 
Leonids, 327-329; the Bielids, 330; orbits of, 331. 

Shower, Meteoric, at L' Aigle, 309 ; the August, 326 ; of the November Leonids, 
327-329; of the Bielids, 330 ; orbits of, 331 ; how to observe a, 333. 

Sidereal Time, 130, 131, 466. 

Sidereal Year defined, 113, 

Signs of the zodiac, 100. 

Sirian stars, 353, 354. 

Sirius, its distance, 349; color, 352 ; spectrum, 353 ; double, 370, 417. 

Small Circles defined, 120. 

22 



338 INDEX. 

Solar stars, 353, 354. 

Solstices defined, 99. 

Spectra, classes of stellar, 353 ; of the nebulae, 388. 

Spectroscope, description of, 69-72. 

Spectrum, the solar, 74. 

Spectrum Analysis, the laws of, 73. 

Sphere, the celestial, 15-17; position of points on, 120. 

Spica, 369. 

Spiral Nebula, 391 ; real form of, 392; location, 416. 

Square of Pegasus, 448. 

Stability, of the planetary system, 186; of Saturn's rings, 262. 

Standard Time, 134. 

Star Finder, 465. 

Star Light, 355. 

Star of Bethlehem, 201. 

Stars, fixed, defined, 1 ; morning and evening, 191, 201 ; number visible, 336; 
scintillation, 337 ; Milky Way, 338 ; tree-like structures, 339 ; constellations, 
340 ; names, 341 ; orders of brightness, 342 ; magnitudes, 343 ; catalogues, 
344; charts, 345 ; distribution, 346 ; clusters, 347 ; dimensions and nature, 
348 ; distances, 349; parallax, 350, 351 ; colors, 352; spectra, 353 ; of differ- 
ent spectral types, 354 ; light and heat, 355 ; the stellar system, 356, 357; 
proper motions. 358; groups, 359; motions in the line of sight, 360; the 
system of, 363 ; double and multiple, 364-372 ; variable, 373-383 ; nebulous, 
385 ; hour angle at any instant, 467 ; list of double, 469 ; list of variable, 469 ; 
list of colored, 469 ; proper names of, 470. 

Stationary Point of a planet's path, 190. 

Structure, of the Milky Way, 338; of the stellar universe, 356, 357. 

Structures, tree-like, 339. 

Sun, distance and diameter, 49; how to view, 50, 51; photosphere, 52 ; rice- 
grains, 53 ; faculae, 54 ; general appearance of a spot, 55 ; changes in appear- 
ance of spots, 56 ; dimensions of spots, $y ; rotation, 58 ; periodicity of the 
spots, 59; observation by Carrington and Hodgson, 60; disturbance on July 
15, 1892, 61 ; cyclonic motion of spots, 62 ; nature of spots, 63; causes of 
weather changes, 64; magnetic storms, 65, 66 ; the storm of February, 1892, 
67 ; frequency of magnetic storms, 68 : the solar spectrum, 74 ; constituents 
of, 75 ; chromosphere, 76 ; prominences. 77-79 ; appearance of the corona, 80 ; 
Schaeberle's theory of the corona, 81 : nature of the corona, 82 ; light, 83 ; heat, 
84 ; causes of radiation, 85 ; contraction theory, 86 ; past and future, 87 ; con- 
stitution, 88; the midnight, 106: enters Aries, 112; the mean, 125; cause 
of eclipses, 173 ; varieties of eclipses, 175 ; partial, annular, and total eclipses, 
176-178; duration and number of eclipses, 179; its path, 361 ; testimony to 
the nebular hypothesis, 402. 

Sun Spots. (See under Sun, 55-64.) 

Superstitions about comets, 299. 

Synodic Period, of the moon, 146; of a planet, 192. 



■™i 



index. 339 

System, stability of the planetary, t86; of the stars, 363; systems of planets 

attending upon stars, 371 ; planetary, 401. 
Swift, Lewis, claims discovery of intra-Mercurial planets, 178 ; his bright comet 

of 1892, 305. 
Swift, the satirist, writes of the moons of Mars, 222, note. 

Tails of comets, 291 ; evolution of, 294; types of, 295 ; of Swift's comet, 305; of 

Brooks's comet shattered, 307. 
Taurus, 459. 

Telegraph, used in determining longitude, 142. 
Telescope, management of, 47 ; reflecting, 25, 41, 42, 43 ; refracting, 25, 34, 42 ; 

equatorial, 44-46 ; Lick, 48 ; invention of, 473. 
Telescopic Objects, 469. 
Telluric Lines, in a spectrum, 75. 
Tempel's comet, 332. 
Temperature at the moon, 166. 

Temporary Stars, defined, 374; Tycho's, 375; Nova Aurigae, 381. 
Terminator of the moon, 1 54. 
Theta Orionis, 372 ; condensed from a nebula, 386 ; spectrum, 398 ; position 

of, 447. 
Thomson, Sir William, on the future of the sun, 87. 
Tides, 169. 
Time, the years, 113; the calendars, 114, 115; mean and apparent solar days, 

125; inequalities of apparent solar days, 126, 127 ; sidereal day, 128 ; civil 

and astronomical day, 129; mean solar and sidereal compared, 130; relation 

between sidereal time and right ascension, 131; longitude and time, 132; 

where the date changes, 133 ; standard, 134; determination of, by a meridian 

circle, 138; sidereal at any instant, 466. 
Tisserand, investigates comet groups, 286. 
Titan, 263. 

Topics for Essays, 474. 
Total Eclipses, lunar, 172; solar, 177, 178. 
Train of a meteor, 313, 315. 

Transits, of Mercury, 197 ; of Venus, 204; of Jupiter's satellites, 248. 
Triangulum, 460. 
Trifid Nebula, 387, 391. 
Tropical Year defined, 113. 

Tschermak, theory of the origin of meteorites, 320. 
Twilight, 118. 
Twinkling of the stars, 337. 
Tycho Brahe, his star, 375. 

Ultra-Neptunian planets, 277. 

Umbra, of a sun spot, 55 ; of a shadow, 170, 174. 

Universe, future of the visible, 404. 



340 INDEX. 

Uranus, discovery, 265 ; distance and diameter, 266 ; revolution and rotation, 267 ; 
appearance, 268; satellites, 269; physical condition, 270; captures the Leo- 
nids, 329 ; captures Tempers comet, 332. 

Ursa Major, 10, 406, 461. 

Ursa Minor, 10, 406, 462. 

Variable Stars, definition, number, names, 373 ; classes, 374 ; temporary stars, 
375; Mira, 376; Class III., 377; Beta Lyras, 378; Algol, 379; Y Cygni, 
380; Nova Aurigae, 381 ; causes of variability, 382; how to observe, 383; 
list of, 469. 

Velocity, of light, 83 ; of meteors, 314 ; of shooting stars, 323 ; of the stars, 360. 

Venus, morning and evening star, 201 ; distance and diameter, 202 ; revolution 
and rotation, 203 ; transits, 204 ; phases and maximum brightness, 205 ; tele- 
scopic appearance, 206; atmosphere, 207; physical condition, 208; transit 
first observed in 1639, 473- 

Vernal Equinox defined, 98. 

Vertical Circles defined, 121. 

Vespasian jokes about a comet, 299. 

Vesta, discovered, 226; diameter, 230. 

Virgo, 463. 

Visual Angle defined, 31. 

Volcanoes, lunar, 157, 158; terrestrial, 400. 

Voltaire writes of the moons of Mars, 222, note. 

Vulpecula, 464. 

Watches, 135. 

Water on the moon, 163, 164. 

Watson, J. C, claims discovery of intra-Mercurial planets, 178; leaves a fund 

for the computation of the orbits of asteroids, 228. 
Weather, changes due to sun spots, 64 ; effect of the moon on, 168. 
Wolf, Max, photographs tree-like structures in the Milky Way, 339. 
Wolf-Rayet Stars, 353, 354, 399. 

Y Cygni. 380. 

Years, different kinds of, 113. 

Young, C. A., theory of sun spots, 63; observation of prominences, 79 ; opinion 

about the collision of a comet with the sun, 300; estimate of the light of the 

stars, 355. 

Zenith defined, 22. 

Zenith Distance defined, 121. 

Zeta Cancri, 372. 

Zodiac defined, 100. 

Zodiacal Light, 334. 



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